See the following from Viewing & Changing Orders - Shopping Help - Apple
Cancel Your Order
If your item is in Processing, you can cancel it online before it ships. Go to your Order Listing
(Opens in a new window)
 page and select an item to see your order details, then select Cancel Item.
Once you’ve submitted your cancellation request, the status message of that item will read Canceled or Pickup Canceled. If you canceled an item that was shipping to a physical address, you will not be billed for your item. If you canceled a pickup item, you may have been billed. We will initiate your refund immediately after you submit your cancellation request.
If you are unable to cancel your item online, you may be able to return it. To see if your item is eligible for a return, refer to the Returns & Refunds help page or speak with an Apple Specialist at 1‑800‑MY‑APPLE.