You can change your payment method for purchases in the settings of your account.

For the App Store:
1. Open your device settings > iTunes Store or App Store.
2. Tap on your Apple ID and choose View Apple ID. If necessary, log in to the system.
3. Choose Payment & Shipping. If you use the Family Sharing function, payment information may only be updated by the group organizer.
4. Change or delete the current information. To delete a payment method, tap on the Delete (-) button.
5. Tap the Ready button.

Please note: when you change your payment method in the iTunes Store or App Store, a small amount of funds may be blocked on your account. This is necessary to check your updated account details.

For Google Play:
1. Open the Google Play Market app.
2. Tap on the menu button > Payments and subscription > Other payment settings.
3. If necessary, log in to your Google Payments account.
4. Under the chosen payment method, tap Change.
5. Enter your changes and tap Update.