The first payment card you add to Apple Pay on a device becomes your default card for that device. If you add more cards, you can change your default card.
To change the default card, follow these steps:
1. Open the Wallet app.
2. Hold down the payment card that you want to set as the default.
3. Drag the card downwards in order to bring it before your other cards, and upon successful completion of the process, you will get a notification that it was successfully set.