You should be able to edit your payment method in your account settings.
For iOS:
1. Open device settings -> iTunes & App Store.
2. Tap your Apple ID email and select View Apple ID. You may be asked to log into your account.
3. Select Payment Information.
4. Edit or remove the information. Tap None to remove a payment method.
5. Tap Done.
For Android:
1. Open Play Store.
2. Tap menu -> Account.
3. Choose Payment methods -> More payment settings.
4. Enter your Google Payments account if asked to do so.
5. Tap Menu in the browser -> Payment methods.
6. Tap Edit under the selected payment method.
7. Apply the changes and tap Save.