follow these instructions: https://customers.gumroad.com/article/216-how-do-i-update-my-credit-card-information-on-gumroad
Eager to load up on Air Miles with your shiny new credit card? Let's do it!
Go to the Billing section of your Settings menu. Click Use a different card. And wait 5 seconds.
You can now enter your credit card information or connect your PayPal account to your Gumroad account for fast future purchases.
When you're good to go, click Update account details