How do I change font of all texts in all scenes?

Late reply.

Here is a an editor window script I made to Find and Replace every UnityEngine.UI.Text fonts in open scene(s) with the option to include project prefabs. (Tested on Unity 5.6.0f3)

If you don't specify a fond to find it will match them all.

Go to Utilities > Replace Font... to use it!

 using System.Collections.Generic;
 using System.Linq;
 using UnityEditor;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 using UnityEngine.UI;
 
 public class FontReplacer : EditorWindow
 {
     private const string EditorPrefsKey = "Utilities.FontReplacer";
     private const string MenuItemName = "Utilities/Replace Fonts...";
 
     private Font _src;
     private Font _dest;
     private bool _includePrefabs;
 
     [MenuItem(MenuItemName)]
     public static void DisplayWindow()
     {
         var window = GetWindow<FontReplacer>(true, "Replace Fonts");
         var position = window.position;
         position.size = new Vector2(position.size.x, 151);
         position.center = new Rect(0f, 0f, Screen.currentResolution.width, Screen.currentResolution.height).center;
         window.position = position;
         window.Show();
     }
 
     public void OnEnable()
     {
         var path = EditorPrefs.GetString(EditorPrefsKey + ".src");
         if (path != string.Empty)
             _src = AssetDatabase.LoadAssetAtPath<Font>(path) ?? Resources.GetBuiltinResource<Font>(path);
 
         path = EditorPrefs.GetString(EditorPrefsKey + ".dest");
         if (path != string.Empty)
             _dest = AssetDatabase.LoadAssetAtPath<Font>(path) ?? Resources.GetBuiltinResource<Font>(path);
 
         _includePrefabs = EditorPrefs.GetBool(EditorPrefsKey + ".includePrefabs", false);
     }
 
     public void OnGUI()
     {
         EditorGUI.BeginChangeCheck();
         EditorGUILayout.PrefixLabel("Find:");
         _src = (Font)EditorGUILayout.ObjectField(_src, typeof(Font), false);
 
         EditorGUILayout.Space();
         EditorGUILayout.PrefixLabel("Replace with:");
         _dest = (Font)EditorGUILayout.ObjectField(_dest, typeof(Font), false);
 
         EditorGUILayout.Space();
         _includePrefabs = EditorGUILayout.ToggleLeft("Include Prefabs", _includePrefabs);
         if (EditorGUI.EndChangeCheck())
         {
             EditorPrefs.SetString(EditorPrefsKey + ".src", GetAssetPath(_src, "ttf"));
             EditorPrefs.SetString(EditorPrefsKey + ".dest", GetAssetPath(_dest, "ttf"));
             EditorPrefs.SetBool(EditorPrefsKey + ".includePrefabs", _includePrefabs);
         }
         
         GUI.color = Color.green;
         if (GUILayout.Button("Replace All", GUILayout.Height(EditorGUIUtility.singleLineHeight * 2f)))
         {
             ReplaceFonts(_src, _dest, _includePrefabs);
         }
         GUI.color = Color.white;
     }
 
     private static void ReplaceFonts(Font src, Font dest, bool includePrefabs)
     {
         var sceneMatches = 0;
         for (var i = 0; i < SceneManager.sceneCount; i++)
         {
             var scene = SceneManager.GetSceneAt(i);
             var gos = new List<GameObject>(scene.GetRootGameObjects());
             foreach (var go in gos)
             {
                 sceneMatches += ReplaceFonts(src, dest, go.GetComponentsInChildren<Text>(true));
             }
         }
 
         if (includePrefabs)
         {
             var prefabMatches = 0;
             var prefabs =
                 AssetDatabase.FindAssets("t:Prefab")
                     .Select(guid => AssetDatabase.LoadAssetAtPath<GameObject>(AssetDatabase.GUIDToAssetPath(guid)));
             foreach (var prefab in prefabs)
             {
                 prefabMatches += ReplaceFonts(src, dest, prefab.GetComponentsInChildren<Text>(true));
             }
 
             Debug.LogFormat("Replaced {0} font(s), {1} in scenes, {2} in prefabs", sceneMatches + prefabMatches, sceneMatches, prefabMatches);
         }
         else
         {
             Debug.LogFormat("Replaced {0} font(s) in scenes", sceneMatches);
         }
     }
 
     private static int ReplaceFonts(Font src, Font dest, IEnumerable<Text> texts)
     {
         var matches = 0;
         var textsFiltered = src != null ? texts.Where(text => text.font == src) : texts;
         foreach (var text in textsFiltered)
         {
             text.font = dest;
             matches++;
         }
         return matches;
     }
 
     private static string GetAssetPath(Object assetObject, string defaultExtension)
     {
         var path = AssetDatabase.GetAssetPath(assetObject);
         if (path.StartsWith("Library/", System.StringComparison.InvariantCultureIgnoreCase))
             path = assetObject.name + "." + defaultExtension;
         return path;
     }
 }

Not sure if there's something in the editor tha can do it but you can swap them out when the game loads with this.

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class TextChanger : MonoBehaviour {
 
     private Text[] GetText;
     public Font myFont;
 
     // Use this for initialization
     void Start () {
         GetText = Text.FindObjectsOfType<Text> ();
 
         foreach (Text go in GetText)
             go.font = myFont;
     }
 }

It's not very good practice though so if someone knows an editor trick to change them go with that.

EDIT - forgot to say, you need that script on any gameObject in each and every scene. It runs when the scene starts so if you have a prefab that instantiates later it may not catch that.

Thanks for the script.

Note to add:

 UnityEditor.SceneManagement.EditorSceneManager.MarkSceneDirty(scene);

So you can then Save the changes to the scene.

Thanks for the Script, I updated it for TextMeshPro ;)

 using System.Collections.Generic;
 using System.Linq;
 using UnityEditor;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 using TMPro;
 
 public class TMProFontReplacer : EditorWindow
 {
     private const string EditorPrefsKey = "Utilities.TMProFontReplacer";
     private const string MenuItemName = "Utilities/Replace TMProFonts...";
 
     private TMP_FontAsset _src;
     private TMP_FontAsset _dest;
     private bool _includePrefabs;
 
     [MenuItem(MenuItemName)]
     public static void DisplayWindow()
     {
         var window = GetWindow<TMProFontReplacer>(true, "Replace TMProFonts");
         var position = window.position;
         position.size = new Vector2(position.size.x, 151);
         position.center = new Rect(0f, 0f, Screen.currentResolution.width, Screen.currentResolution.height).center;
         window.position = position;
         window.Show();
     }
 
     public void OnEnable()
     {
         var path = EditorPrefs.GetString(EditorPrefsKey + ".src");
         if (path != string.Empty)
             _src = AssetDatabase.LoadAssetAtPath<TMP_FontAsset>(path) ?? Resources.GetBuiltinResource<TMP_FontAsset>(path);
 
         path = EditorPrefs.GetString(EditorPrefsKey + ".dest");
         if (path != string.Empty)
             _dest = AssetDatabase.LoadAssetAtPath<TMP_FontAsset>(path) ?? Resources.GetBuiltinResource<TMP_FontAsset>(path);
 
         _includePrefabs = EditorPrefs.GetBool(EditorPrefsKey + ".includePrefabs", false);
     }
 
     public void OnGUI()
     {
         EditorGUI.BeginChangeCheck();
         EditorGUILayout.PrefixLabel("Find:");
         _src = (TMP_FontAsset)EditorGUILayout.ObjectField(_src, typeof(TMP_FontAsset), false);
 
         EditorGUILayout.Space();
         EditorGUILayout.PrefixLabel("Replace with:");
         _dest = (TMP_FontAsset)EditorGUILayout.ObjectField(_dest, typeof(TMP_FontAsset), false);
 
         EditorGUILayout.Space();
         _includePrefabs = EditorGUILayout.ToggleLeft("Include Prefabs", _includePrefabs);
         if (EditorGUI.EndChangeCheck())
         {
             EditorPrefs.SetString(EditorPrefsKey + ".src", GetAssetPath(_src, "ttf"));
             EditorPrefs.SetString(EditorPrefsKey + ".dest", GetAssetPath(_dest, "ttf"));
             EditorPrefs.SetBool(EditorPrefsKey + ".includePrefabs", _includePrefabs);
         }
 
         GUI.color = Color.green;
         if (GUILayout.Button("Replace All", GUILayout.Height(EditorGUIUtility.singleLineHeight * 2f)))
         {
             ReplaceFonts(_src, _dest, _includePrefabs);
         }
         GUI.color = Color.white;
     }
 
     private static void ReplaceFonts(TMP_FontAsset src, TMP_FontAsset dest, bool includePrefabs)
     {
         var prefabMatches = 0;
         if (includePrefabs)
         {
             var prefabs =
                 AssetDatabase.FindAssets("t:Prefab")
                     .Select(guid => AssetDatabase.LoadAssetAtPath<GameObject>(AssetDatabase.GUIDToAssetPath(guid)));
             foreach (var prefab in prefabs)
             {
                 prefabMatches += ReplaceFonts(src, dest, prefab.GetComponentsInChildren<TextMeshProUGUI>(true));
                 UnityEditor.EditorUtility.SetDirty(prefab);
             }
         }
 
         var sceneMatches = 0;
         for (var i = 0; i < SceneManager.sceneCount; i++)
         {
             var scene = SceneManager.GetSceneAt(i);
             var gos = new List<GameObject>(scene.GetRootGameObjects());
             foreach (var go in gos)
             {
                 sceneMatches += ReplaceFonts(src, dest, go.GetComponentsInChildren<TextMeshProUGUI>(true));
             }
         }
         UnityEditor.SceneManagement.EditorSceneManager.MarkAllScenesDirty();
 
         if (includePrefabs)
         {
             Debug.LogFormat("Replaced {0} font(s), {1} in scenes, {2} in prefabs", sceneMatches + prefabMatches, sceneMatches, prefabMatches);
         }
         else
         {
             Debug.LogFormat("Replaced {0} font(s) in scenes", sceneMatches);
         }
     }
 
     private static int ReplaceFonts(TMP_FontAsset src, TMP_FontAsset dest, IEnumerable<TextMeshProUGUI> texts)
     {
         var matches = 0;
         var textsFiltered = src != null ? texts.Where(text => text.font == src) : texts;
         foreach (var text in textsFiltered)
         {
             text.font = dest;
             matches++;
         }
         return matches;
     }
 
     private static string GetAssetPath(Object assetObject, string defaultExtension)
     {
         var path = AssetDatabase.GetAssetPath(assetObject);
         if (path.StartsWith("Library/", System.StringComparison.InvariantCultureIgnoreCase))
             path = assetObject.name + "." + defaultExtension;
         return path;
     }
 }