How to get a refund from things bought with apply pay

If you cannot cancel the money sent and they are not sending it back you’ll need to call Apple to start a dispute.

See “How to cancel or dispute a payment“ at https://support.apple.com/en-us/HT207875

If you're referring to Apple Pay Cash, you don't.

Apple Pay Cash is just like using cash money. There are not protections or ability to dispute or contest charges like there are with a credit card.