How do I add another credit card to Apple Pay?

Add another card in Wallet: -> Tap Wallet -> tap the Plus sign (top right) -> and follow the instructions on the screen.

Tap Wallet -> tap your card -> tap the three dots at the top right of the screen -> scroll down -> tap Remove this card.

Please refer to Change or remove the payment cards that you use with Apple Pay, which explains "You can change your default card, remove a card from Apple Wallet, or update the billing and contact information that Apple Pay uses for in-app and online purchases."