In case you need to change your payment method or even add a new payment method to your account you can do it easily by following these steps:
1. Select what you want to purchase.
2. A pop up will appear with the information of the item or package you selected. You can then see and select your payment method.
3. From the option shown you can change your payment method or choose a voucher if you have one.
On the other hand, if you use an Apple device, please visit the following page: https://support.apple.com/en-us/HT201266
We hope this helps you out! :)