You can cancel the order within 30 minutes after placing it from your Order Status page, see this link: http://www.apple.com/shop/help/print_products
If you want to cancel an order, then you may do so within 30 minutes after checkout by logging into online Order Status with your Apple ID and password.
After logging in, locate the order you want to cancel, and click "Cancel Order" in the Status column. You'll be asked to confirm your request to cancel the order. Once you confirm the request, a cancellation confirmation email will be sent to the email address in your Apple ID account.
If your shipping address is incorrect, and you are unable to cancel your order within 30 minutes, your order will be shipped to the address provided. If the carrier is unable to deliver your order to the address provided, they will return the package to Apple and we will provide a full refund.
If you are past the time, where you can cancel the order, contact Apple Support from this page: http://www.apple.com/shop/help/print_products/routing