The first card you add to Google Pay automatically becomes your default card.

You can change your default card two ways:

On your device

1. Open the Google Pay app.
2.  Select the card you want as the default a card, then touch “Set as default card”.

Alternatively, touch and drag a card into the "default card" spot at the top of the Google Pay Home screen.

Via the Google accounts webpage

1. Sign in to your account at accounts.google.com.
2.  On the left-hand side, click “Payment methods”.
3. To the right of your chosen card, click “Set as default”.