Input.GetMouseButtonDown no working

i have a get mouse button down script for an sprite for when i click the 2d sprite a level loads. I though it worked but it turns out everywhere i click i load the scene. How do i make it so when i click on the sprite on its own a scene loads.

    function Update () {

Input.GetMouseButtonDown(0) works just how it should be.

I would recommend you to watch this tutorial