How do I update my billing address?

Click on the My Account link at the top right of the screen.

MyAccountLink

You will find yourself on your profile page.  Below your account information, you will see your default billing address and shipping address from your address book.  Click on the "Edit Address" link under your default billing address.

EditBilling1

In the screen that opens, enter your new address information and click on the blue "Save Address" button.

EditBilling2