first make a main texture "textureButton" then assign that texture " Texture1" in the awake function. Use the 3rd texture "Texture2" when you want to swap it

var Texture1 : Texture; var Texutre2 : Texture; var TextureButton: Texture;

function Awake() { TextureButton = Texture1; }

void OnGUI() {

if (GUI.Button (new Rect (50,410,60,60),  TextureButton)) {
         TextureButton = Texture2;   //Swap to Texture2
        }