If you need to edit an order, you can click the edit order button from your order confirmation page:
Your original order will be canceled and your account will be credited with the amount you paid.
The parts you ordered will be returned to your cart. From the cart you can add more parts or remove the parts you no longer need.
When you proceed to the checkout, you'll be able to choose the shipping address & your delivery speed options for your new order.
You'll see the account credit show on the right hand side of the page:
Unused credit will be stored on your account for your next order.