You can reverse a transaction made with Apple Pay just like you would with a physical card. Please contact your merchant if you wish to request a reversal or refund.
You may be asked to provide the last four digits of your Device Account number. To find this on your device, selectSettings ➡ Wallet & Apple Pay ➡ Card ➡scroll down and view 'Device Account Number'. You can also find it in the Wallet app by selecting the card and tapping on the symbol with the 3 dots on the lower right hand corner and then scrolling down to view 'Device Account Number'.