There are a few ways to cancel a transaction. You can cancel by tapping the No Sale button:
You will then see the confirmation screen pop up. If you are ready to cancel this transaction, tap Yes, indicating that you are sure:
After your transaction is cancelled with the No Sale button, you can begin your next transaction.
Another way to cancel a transaction, if you are already in the process, is to tap Cancel on the upper left corner of any menu where you are to enter payment info. This will also immediately cancel the transaction so you can start over: