How do I update my payment method for in app purchases?

Please review the Apple Support article I linked below. You need to change the order of the payment methods in your Apple ID Account (not your Wallet app).

>>Reorder payment methods on your iPhone, iPad, or Apple Vision Pro

1. On the Payment & Shipping screen, tap Edit.
2. Touch and hold a payment method to drag it further up or down your list of payment methods. Apple will attempt to charge your payment methods in the order that they appear.
3. Tap Done.<<

If you already have a payment method on file, learn how to change or update your payment method.

Add a payment method - Apple Support

How apps, content, and subscriptions from Apple are billed - Apple Support

Tap the Apple Card with your finger when it appears. Then you’ll see the other cards in your Wallet app. Tap the card you want to use for payment method.