To cancel an unpaid order:
  1. Visit My Orders 
  2. Click the item
  3. Click More
  4. Choose Cancel Order, select a reason, add an optional note, then click Cancel Order
If you are using the app and have an unpaid item, you'll need to click Cancel first, then go to the menu and follow the steps above.

If you already paid, see How can I get a refund on an order?