When you have a project in your cart you want to change, click 'edit' by the project in your cart to change it. 

This will take you back to the project so you can make your changes and then click "Go to Cart" in the upper righthand corner once ready to add the new version to the cart.  

Click "Remove" if you'd like to take the project out of the cart. 

Click Make a Copy to make a copy of your project under the My Projects page.