You can change your order on Gamefound yourself as long as the address change deadline has not passed; this date will be announced in the project's updates. To do this:
1 - Log into gamefound.com using the email address associated with your pledge
2 - Go to "My Backed Projects" in your account
3 - Find the project in question and click "View Order"
4 - From inside the order, you'll be able to add new items to your order, generally by navigating back to the main page of the pledge manager and adding individual items to your cart. When you return to your cart, you'll be asked to pay the balance due.