Need to alter script to load next level when level is completed

I'm having some trouble here. I'm really new at coding but trying to learn with a sample project. I have this demo script here for my game manager but I need to alter it so that when the player completes a level they can progress to the next one.

Can someone give me a hand with this?

 using System.Collections;
 using UnityEngine;
 
 //List of all the posible gamestates
 public enum GameState
 {
     NotStarted,
     Playing,
     Completed,
     Failed
 }
 
 //Make sure there is always an AudioSource component on the GameObject where this script is added.
 [RequireComponent(typeof(AudioSource))]
 public class GameManager : MonoBehaviour
 {
     //Text element to display certain messages on
     public GUIText FeedbackText;
 
     //Text to be displayed when entering one of the gamestates
     public string GameNotStartedText;
     public string GameCompletedText;
     public string GameFailedText;
 
     //Sounds to be played when entering one of the gamestates
     public AudioClip StartSound;
     public AudioClip FailedSound;
 
     private GameState currentState = GameState.NotStarted;
     //All the blocks found in this level, to keep track of how many are left
     private Block[] allBlocks;
     private Ball[] allBalls;
 
     // Use this for initialization
     void Start()
     {
         Time.timeScale = 1;
 
         //Find all the blocks in this scene
         allBlocks = FindObjectsOfType(typeof(Block)) as Block[];
 
         //Find all the balls in this scene
         allBalls = FindObjectsOfType(typeof(Ball)) as Ball[];
 
         //Prepare the start of the level
         SwitchTo(GameState.NotStarted);
     }
 
     // Update is called once per frame
     void Update()
     {
         switch (currentState)
         {
             case GameState.NotStarted:
                     //Check if the player taps/clicks.
                     if (Input.GetMouseButtonDown(0))    //Note: on mobile this will translate to the first touch/finger so perfectly multiplatform!
                     {
                         for (int i = 0; i < allBalls.Length; i++)
                             allBalls[i].Launch();
 
                         SwitchTo(GameState.Playing);
                     }
                 break;
             case GameState.Playing:
                 {
                     bool allBlocksDestroyed = true;
 
                     //Check if all blocks have been destroyed
                     for (int i = 0; i < allBlocks.Length; i++)
                     {
                         if (!allBlocks[i].BlockIsDestroyed)
                         {
                             allBlocksDestroyed = false;
                             break;
                         }
                     }
 
                     //Are there no balls left?
                     if (FindObjectOfType(typeof(Ball)) == null)
                         SwitchTo(GameState.Failed);
 
                     if (allBlocksDestroyed)
                         SwitchTo(GameState.Completed);
                 }
                 break;
             //Both cases do the same: restart the game
             case GameState.Failed:
             case GameState.Completed:
                 //Check if the player taps/clicks.
                 if (Input.GetMouseButtonDown(0))    //Note: on mobile this will translate to the first touch/finger so perfectly multiplatform!
                     Restart();
                 break;
         }
     }
 
     //Do the appropriate actions when changing the gamestate
     public void SwitchTo(GameState newState)
     {
         currentState = newState;
 
         switch (currentState)
         {
             default:
             case GameState.NotStarted:
                 DisplayText(GameNotStartedText);
                 break;
             case GameState.Playing:
                 audio.PlayOneShot(StartSound);
                 DisplayText("");
                 break;
             case GameState.Completed:
                 audio.PlayOneShot(StartSound);
                 DisplayText(GameCompletedText);
                 StartCoroutine(RestartAfter(StartSound.length));
                 break;
             case GameState.Failed:
                 audio.PlayOneShot(FailedSound);
                 DisplayText(GameFailedText);
                 StartCoroutine(RestartAfter(FailedSound.length));
                 break;
         }
     }
 
     //Helper to display some text
     private void DisplayText(string text)
     {
         FeedbackText.text = text;
     }
 
     //Coroutine which waits and then restarts the level
     //Note: You need to call this method with StartRoutine(RestartAfter(seconds)) else it won't restart
     private IEnumerator RestartAfter(float seconds)
     {
         yield return new WaitForSeconds(seconds);
 
         Restart();
     }
 
     //Helper to restart the level
     private void Restart()
     {
         Application.LoadLevel(0);
     }
 }