You cannot modify an order once placed -- you either need to call and see if the part is available for purchase separately or cancel the existing order and submit a new order.  
Hi I was checking my order and my Aurora r13 doesn't  have the back cover, so is there any way to get it without cancel my order? please. Also I don't want modify it my order because I don't want to interfere with the time process for its production. Thank you!
Aurora R13 Back Cover