If you have auto-subscription enabled through Fastspring, you then have the option to change the billing address, the payment method or update the credit card. 

1. After the first payment is completed, we send a payment receipt that has the subscription link:

2. Click on the link to open the subscription page and click on "Edit Subscription"

3. You will then get the option to edit the billing address or the payment method

Using the "Update Payment Method" you can change the payment method or update the credit card.