How do I cancel an Apple Pay payment

See the following from Send, receive, and request money with Apple Pay - Apple Support

If you send money to someone who hasn't accepted the payment, you can cancel the payment. Here's how:

1. Open Messages and go to the payment. Or go to the payment in transaction history. 
2. Tap the payment to see its details, then check the Status field. 
3. Tap Cancel Payment. The Status field updates to say Canceled. If you
4. don't see the option to cancel, then the person has already accepted
5. the payment. Try asking the person to send money back to you instead. 

If you reject or cancel a payment, the money will be refunded by the next business day depending on your bank or card issuer. 

Also from the above link:

"How to dispute a payment

To open a dispute for a transaction, contact us. Apple Pay person to person payments are just like any other private transaction between two people. If you sent money to the wrong person, try to cancel the payment or ask them to send the money back."

It's a credit or debit card payment so best contact the card company.