Unity UI How to make Multiple Clickable Next / Back Buttons?

What some terrible code... Here is a complete rework I can suggest you. It involves more classes, but is way more extensible and manageable that what you have right now.

CODE NOT TESTED

NextBackButton.cs

 using System.Collections;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class NextBackButton: MonoBehaviour
 {
     [SerializeField] private Button previousButton;
     [SerializeField] private Button nextButton;
     [SerializeField] private Text previousButtonLabel;
     [SerializeField] private Text nextButtonLabel;
     private int currentOption = 0;
     private ButtonListener[] listeners = new ButtonListener[]
     {
         new Foo(), new Bar(), new Baz("hello world")
     };
 
     private void Start()
     {
         SetListeners(0);
         previousButton.onClick.AddListener(() => SetListeners(currentOption - 1));
         nextButton.onClick.AddListener(() => SetListeners(currentOption + 1));
     }
 
     void SetListeners(int index)
     {
         listeners[currentOption].RemoveAsListener();
         listeners[(currentOption - 1 + listeners.Length) % listeners.Length].RemoveAsListener();
 
         currentOption = (index + listeners.Length) % listeners.Length;
 
         previousButton.interactable = currentOption > 0;
         if (previousButton.interactable)
         {
             listeners[(currentOption - 1 + listeners.Length) % listeners.Length].AddAsListener(previousButton);
             listeners[( currentOption - 1 + listeners.Length ) % listeners.Length].SetLabel(previousButtonLabel);
         }
         else
         {
             previousButtonLabel.text = "";
         }
 
         nextButton.interactable = currentOption < listeners.Length - 1;
         if (nextButton.interactable)
         {
             listeners[currentOption].AddAsListener(nextButton);
             listeners[currentOption].SetLabel(nextButtonLabel);
         }
         else
         {
             nextButtonLabel.text = "";
         }
     }
 }

ButtonListener.cs

 public abstract class ButtonListener
 {
     protected Button button;
 
     public abstract void SetLabel(Text text);
 
     public virtual void AddAsListener(Button button)
     {
         this.button = button;
         button.onClick.AddListener(PerformAction);
     }
 
     public virtual void RemoveAsListener()
     {
         if(button == null) return;
         button.onClick.RemoveListener(PerformAction);
     }
 
     protected abstract void PerformAction();
 }

Foo.cs = your RunCode0 function

 public class Foo : ButtonListener
 {
     public override void SetLabel(Text text)
     {
         text.text = "Foo";
     }
 
     protected override void PerformAction()
     {
         Debug.Log("Hey, I'm foo!");
     }
 }

Bar.cs = your RunCode1 function

 public class Bar : ButtonListener
 {
     private int selectedCount = 0;
 
     public override void SetLabel(Text text)
     {
         text.text = "Bar";
     }
 
     public override void AddAsListener(Button button)
     {
         base.AddAsListener(button);
         Debug.Log("Listener added"); // Just for the sake of the example, you don't need the function
     }
 
     protected override void PerformAction()
     {
         selectedCount++;
         Debug.Log("This option has been clicked " + selectedCount + " times");
     }
 }

Baz.cs = your RunCode2 function

 public class Baz : ButtonListener
 {
     private string input;
     public Baz(string input)
     {
         this.input = input;
     }
 
     public override void SetLabel(Text text)
     {
         text.text = input;
     }
 
     protected override void PerformAction()
     {
         Debug.Log(input);
     }
 }

Change your myButtonClick() to myButtonClick(int n). When in the On Click() for the Button, when you pick NextBackButton > myButtonClick you'll see it request an int value. Set this to 1 for your forwards button and to -1 for your back button and use it to adjust a counter.
Use the counter in myButtonClick to keep track of thing and test the count so that it doesn't go below 0 or greater than 3.

Finally, this works. Here's my code;

NextBackButton.cs

  using System.Collections;
  using System.Collections.Generic;
  using UnityEngine;
  using UnityEngine.UI;
  
  public class NextBackButton: MonoBehaviour
  {
      [Range(0, 3)] public int currentOption = 0;
  
      [SerializeField] private Button previousButton;
      [SerializeField] private Button nextButton;
  
      private void Start()
      {
          // set the initial text of the button
          //setButtonText(currentOption);
  
          // add an event listener to look out for button clicks
         previousButton.onClick.AddListener(() => myButtonClick(currentOption--));
         nextButton.onClick.AddListener(() => myButtonClick(currentOption++));
      }
 
      private void Update()
      {
         // Minimum Value
         if (currentOption < 0)
         {
             currentOption = 0;
         }
 
         // Maximum Value
         if (currentOption > 3)
         {
             currentOption = 3;
         }
      }
  
      public void myButtonClick(int optionValue)
      {
          switch (currentOption)
          {
              case 0:
                  // run stuff for option 1
                  RunCode0();
  
                  // change the text on the button to be the next option
                  optionValue++;
 
                  //setButtonText(currentOption);
                  break;
  
              case 1:
                  Debug.Log("Doing option 2 things");
                  RunCode1();
  
                  // change the text on the button to be the next option
                  optionValue++;
                  
                  //setButtonText(currentOption);
                  break;
  
              case 2:
                  Debug.Log("Doing option 3 things");
                  RunCode2();
  
                   // change the text on the button to be the next option
                  optionValue++;
 
                  //setButtonText(currentOption);
                  break;
  
              case 3:
                  Debug.Log("Doing option 4 things");
                  RunCode3();
 
                  // change the text on the button to be the next option
                  optionValue++;
 
                  //setButtonText(currentOption);
                  break;
          }
  
      }
  
      void setButtonText(string buttonText)
      {
          previousButton.transform.GetChild(0).GetComponent<Text>().text = buttonText;
          nextButton.transform.GetChild(0).GetComponent<Text>().text = buttonText;
      }
 
      public void RunCode0()
      {
          //Do something
      }
  
     public void RunCode1()
      {
          //Do something
      }
  
     public void RunCode2()
      {
          //Do something
      }
  
      public void RunCode3()
      {
          //Do something
      }
  }