How to change the alpha of a sprite

I have a script and I want it so that when I cant press a button the the alpha changes to half. my script is:

 using UnityEngine;
 using System.Collections;
 
 public class Play : MonoBehaviour {
 
     public static bool readyToGo;
     public SpriteRenderer me;
 
     void OnMouseDown(){
         if (readyToGo) {
             if (SelectMode.modeSelect == 0) {
                 Application.LoadLevel ("Original");        
             }
             if (SelectMode.modeSelect == 1) {
                 Application.LoadLevel ("Ascension");        
             }
             if (SelectMode.modeSelect == 2) {
                 Application.LoadLevel ("Freefall");        
             }
         }
     }
 
     void Update(){
         if (!readyToGo) {
             //me.sprite.a = 0.5f;        
         }
     }
 }

Does anybody know how to achieve this?