How do I make a button change the index of an array in my script?

Noob here. I am very unsure what's wrong with my code. I'm trying to make it so that the array stops at lines[2]. Then, when a button is pressed, I want it to change the index to an integer that is +1 higher than it originally was or +2 higher if a different button is pressed. I have already kind of figured out how to make it stop at lines[2], but it keeps giving me the IndexOutOfRangeException, even though it functions as intended when I actually play the game. Any help or point in the right direction would be greatly appreciated. Thank you!

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using TMPro;
 
 public class Dialogue : MonoBehaviour
 {
     public TextMeshProUGUI textComponent;
     public string[] lines;
     public float textSpeed;
 
     private int index;
 
     void Start()
     {
         textComponent.text = string.Empty;
         StartDialogue();
     }
     void Update()
     {
         if (Input.GetMouseButtonDown(0))
         {
                 if (textComponent.text == lines[index])
                 {
                     NextLine();
                 }
                 else
                 {
                     StopAllCoroutines();
                     textComponent.text = lines[index];
                 }
         }
         if (textComponent.text == lines[2]) //pause
         {
             index--;
         }
     }
 
     void StartDialogue()
     {
         index = 0;
         StartCoroutine(TypeLine());
     }
 
     IEnumerator TypeLine()
     {
         foreach (char c in lines[index].ToCharArray())
         {
             textComponent.text += c;
             yield return new WaitForSeconds(textSpeed);
         }
     }
     void NextLine()
     {
         if (index < lines.Length - 1)
         {
             index++;
             textComponent.text = string.Empty;
             StartCoroutine(TypeLine());
         }
         else
         {
             gameObject.SetActive(false);
         }
     }
     public void choice1()
     {
         index = 3;
     }
     public void choice2()
     {
         index = 4;
     }
 }
 //built with "5 Minute DIALOGUE SYSTEM in UNITY Tutorial" by BMo