i am trying switch case Randomly switch every second and i am trying to create a start button for my time ,if everyone can help please do thanks

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class ChangeImages : MonoBehaviour
 {
 
    int  n;
     int n2;

 public Image imageView1;
 public Image imageView2;
 public Text timerLabel;
 public float mycoolTimer = 3;
 private Text TimerText;

  
 void Start()
 {
     TimerText = GetComponent<Text>();
 }



 void Update()
 {
     mycoolTimer -= Time.fixedDeltaTime;
     TimerText.text = mycoolTimer.ToString("f0");
     print(mycoolTimer);
     if (mycoolTimer < 0)
     {
         Time.timeScale = 0;
     }
 }
 void TimerUpdate(){  
     
           
     n = Random.Range(0, 4);
     switch (n)
     {
         case 0:
             imageView1.sprite = Resources.Load<Sprite>("Image/blu") as Sprite;

             break;
         case 1:
             imageView1.sprite = Resources.Load<Sprite>("Image/n1") as Sprite;
             break;
         case 2:
             imageView1.sprite = Resources.Load<Sprite>("Image/n2") as Sprite;
             break;

         case 3:
             imageView1.sprite = Resources.Load<Sprite>("Image/n3") as Sprite;
             break;
         default:
             break;
     }

     n2 = Random.Range(0, 4);
     switch (n2)
     {
         case 0:
             imageView2.sprite = Resources.Load<Sprite>("Image/blu") as Sprite;

             break;
         case 1:
             imageView2.sprite = Resources.Load<Sprite>("Image/n1") as Sprite;
             break;
         case 2:
             imageView2.sprite = Resources.Load<Sprite>("Image/n2") as Sprite;
             break;

         case 3:
             imageView2.sprite = Resources.Load<Sprite>("Image/n3") as Sprite;
             break;
         default:
             break;
     }