How to delete prefabs without warning popping up?

I'm trying to make a very very basic fps game where the player can walk around, point, and un-point his gun, and shoot stuff. The way im trying to do it, I spawn a picture of an unaimed gun in void Start(). Then, if the player presses on the left button of the mouse, I wan't to delete the old picture and replace it with the guy aiming the gun. Here is my code.

using UnityEngine;

using System.Collections;

public class GunAim : MonoBehaviour {

public GameObject NotAiming;

public GameObject Aiming;

public float yPos = -2.15f;

 // Use this for initialization
 void Start ()
 {

 Instantiate(NotAiming, new Vector3(0, yPos, 0), Quaternion.identity);

 }

void Update() {

 if(Input.GetKeyDown(KeyCode.Mouse0))
     {

   Instantiate(Aiming, new Vector3(0, yPos, 0), Quaternion.identity);
         Destroy(NotAiming);

     }

 }

}

Problem is, I get an error when I try to destroy a prefab.