How to change my credit card

It is unclear if you are referring to your Default card used for Apple Pay purchases in your Wallet, or if you are talking about the payment methods you use for your Apple Account for in-app purchases.

For the default card used for Apple Pay in your Wallet, you go to Settings > Wallet & Apple Pay > Default Card, and choose a different one.

For your Apple Account purchases, you would need to first add a new valid payment method, and then remove the old one. You can also change the order of the cards used for purchases on your Apple Account. This is done on your Apple Account, not in the Wallet app.

If you need to change or update your Apple Account payment method - Apple Support