How do I add another credit card to Apple Pay?

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."

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.