How to change sprite OnMouseEnter?

I have a play button sprite that I want to change to another image when your mouse crosses over it, and back when it exits. Then when I click it, I want to move to the next scene, which I know how to do. I know that these functions could be used, but I don't have a clue what would go in them.

 #pragma strict
 
 function OnMouseEnter () {
     
 }
 
 function OnMouseExit (){
 
 }

I'd prefer if you taught me in JS, because that's what I'm learning ATM, and I have no experience with other languages.