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
}