To make in-app purchases, you must have a valid purchase method linked to your Google account, which can be changed, added, removed or edited at any time.

How to Change Purchase Method
1. Run the Google Play app.
2. Tap Menu > Payment methods > More payment settings.
3. Sign in to pay.google.com.
4. Select your new payment method.
5. Select Edit.
6. Tap Update.

To learn how to add, remove, or edit your payment method, please check the following link.
https://support.google.com/googleplay/answer/4646404