change a gameobject into another

I need to transform one object into another one.
I have this object into my menu:
alt text

when i put the mouse cursor on it i want that it becomes like this:
alt text

and than when the user put the mouse cursor out of the object it return to be the button (1st immage).

How can i code this in c#?

Use OnMouseEnter and OnMouseExit.

public GameObject newSprite;
private Vector3 currentSpritePosition;

void OnMouseEnter(){
  //getting the current position of the current sprite if ever it can move;
  currentSpritePosition = transform.position;

  //then make it invisible
  renderer.enabled = false;

  //give the new sprite the position of the latter
  newSprite.transform.position = currentSpritePosition;

  //then make it visible
  newSprite.renderer.enabled = true;
}

void OnMouseExit(){
  //just the reverse process
  renderer.enabled = true;
  newSprite.renderer.enabled = false;
}