To update your payment method:
For iOS:
1. Go to settings (on your phone)
2. Click on Apple ID, iCloud, Media & Purchases (the one on the top of settings)
3. Go to Payment & Shipping
4. Change your default payment method
For Android:
1. Go to Google Play Store app
2. Go to Menu > Payment methods > More payment settings
3. Under payment method you want to edit, tap Edit
4. Change your payment method
5. Tap Update