change prefabs with Gui.buttons

i have a problem with my script i can't replace the prefab, this script no destroy the others prefabs and show a prefabs on other

  MENU WITH BUTTONS 

[cube ] [sphere]

[plane ] [capsule]

this the script:

var thePrefab1: GameObject; var thePrefab2: GameObject; var thePrefab3: GameObject; var thePrefab4: GameObject;

function OnGUI () { // Make a background box GUI.Box (Rect (10,10,190,130), "MENU WITH BUTTONS ");

 // Make the first button. If it is pressed, Application.Loadlevel (1) will be executed
 if (GUI.Button (Rect (20,40,80,20), "Cube")) {
     var instance1 : GameObject = Instantiate(thePrefab1, transform.position, transform.rotation);    
     }

 // Make the second button.

if (GUI.Button (Rect (110,40,80,20), "Sphere")) { var instance2 : GameObject = Instantiate(thePrefab2, transform.position, transform.rotation);

 }
 
 // Make the third button.
  if (GUI.Button (Rect (20,90,80,20), "Capsule")) {
 
     var instance3 : GameObject = Instantiate(thePrefab3, transform.position, transform.rotation);
 }
 
 // Make the second button.
 if (GUI.Button (Rect (110,90,80,20), "cylinder")) {
 
     var instacne4 : GameObject = Instantiate(thePrefab4, transform.position, transform.rotation);
 }
 

}