The first card added to Apple Wallet is the default card. If you add more than one card, you can change the default card at any time. The process is slightly different for each device.
iPhone - Go to Settings > Wallet & Apple Pay, scroll down to Transaction Defaults. Tap Default Card and choose a new card
Apple Watch - On the iPhone, open the Apple Watch app, and go to the My Watch tab. Scroll down and tap Wallet & Apple Pay > Default Card and choose a new card.
Mac with Touch ID - Go to System Preferences > Wallet & Apple Pay. Choose a new card from the Default Card pop-up menu.