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 (long press).
3. Drag the card downwards in order to bring it in front of your other cards.
You will receive a notification once the selection of the default card was successful.