Call a function across several objects

The best approach (in my opinion) is to have a ButtonManager that keeps track of all the buttons instantiated in a list, then you can iterate through the buttons and call the refresh function for each. Keeping references stored somewhere of each button will be the fastest and cleanest way.

Here's some code

     class ButtonManager
     {
       static List<Button> InstantiatedButtons= new List<Button>();
     
       public static void RefreshButtons()
       {
         for (int i=0;i<InstantiatedButtons.Count;i++)
           InstantiatedButtons[i].Refresh();
       }
     
       public static Button CreateButton()
       {
         Button but = new Button();
         InstantiatedButtons.Add(but);
         return but;
       }
     }

You will have to instantiate all Buttons through the ButtonManager thou.

This is one approach. Another would be for each Button on Start/Awake to register themselves into the button Manager.

Another thing that you can do (but it will be slow) is to find all elements of type button from the scene and call the refresh for them.

 Button[] buttons = GameObject.FindSceneObjectsOfType(typeof (Button));
 
 for (int i=0;i<buttons.length;i++)
  buttons[i].Refresh();