You have access within your account to cancel an order after you submit, before it goes into production, while it is still in "Processing."

Once within your portal, you will select to "View orders" and select the order ID you would like to cancel. 

Once you are within that order, you will see a "Tasks" dropdown option under the shipping address on the left hand side of your screen which will allow you to select "Cancel Order."

Although you cannot alter an order, cancelling does provide the option for you to resubmit with your alterations.