Arcade name input with two buttons

Hi! I'm making a highscore board but I'm having trouble with the name input, since I won't have a keyboard I have to work only with two buttons, one would change the character and the other will select it, making a three character string. That would be sent to the highscore board, but that I have figured it out. I have a Text field and I want to show the character when you're changing it, like a classic arcade game. Here's the code I have so far:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class SetName : MonoBehaviour
 {
     public Text aText;
     private int countKey;
     private char[] alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();
     private char[] ABC;
     private bool notSelected;
     private bool doneSelection;
     // Start is called before the first frame update
     void Start()
     {
         notSelected = true;
         doneSelection = false;
         ABC = new char[3];
         countKey = 0;
         StartCoroutine("charName");
     }
 
     /// <summary>
     /// Update is called every frame, if the MonoBehaviour is enabled.
     /// </summary>
     void Update()
     {
         if(doneSelection == true)
         {
         StopCoroutine("charName");
         PlayerPrefs.SetString("PlayerName",ABC.ToString());
         }
     }
 
     IEnumerable charName()
     {
         while(true)
         {
             for(int i = 0; i <= 3; i++)
             {
                 notSelected = true;
                 
                 while(notSelected == true)
                 {
                     if (Input.GetKeyDown(KeyCode.LeftArrow))
                     {
                         if(countKey <= 26)
                             countKey++;
                         else countKey = 0;
 
                         aText.text = alpha[countKey].ToString();
                     }
 
                     if(Input.GetKeyDown(KeyCode.Space))
                     {
                         ABC[i]=alpha[countKey];
                         notSelected = false;
                     }
                 }
             }
 
         doneSelection = true;
         
         }
     }
 }

I know I may have several mistakes but I'll be glad if someone could help me! :)