All in-game purchases are made through the app you used to download and install the game (Google Play or App Store).
You can change your method of payment in the settings section of your registered Apple ID/Google Play.
App Store Users:
1. Open "Settings"
2. Click on your name > "Payment and delivery". You may need to input your Apple ID information.
3. To add a payment method, click on "Add payment method".
4. To renew a payment method, click on it and change the necessary information. You may need to enter a security code listed on your credit or debit card.
5. To remove a payment method, click on "Change", then click on the "Remove" button.
Google Play Users:
1. Open the "Google Play Market" app on your device.
2. Press the Menu icon > Payment methods > Additional payment settings.
3. If necessary, access your account at pay.google.com.
4. Press "Change" under the chosen payment method.
5. Make your changes and press "Renew".
Additional information regarding additions/changes to your payment method is available on the Google Play support section.