How to change a card in Apple Pay

To change your Default card used for Apple Pay:

Change or remove the payment cards that you use with Apple Pay - Apple Support

If you are trying to change a subscription that is currently being billed to a card, that is not being done through Apple Pay.

You select another card in your Apple Wallet. You can set a default card by following this path

iPhone > Settings > Wallet & Apple Pay > Apple Pay Defaults > Default Card > select card from cards saved in Wallet app

As a follow up to Jim’s and my post, here is an Apple Support article on how to change and prioritize cards in your Apple Account (Apple ID).

Add a payment method to your Apple Account - Apple Support