You can absolutely change your billing method!
To change the billing method for your existing hint subscription:
Login into your account at drinkhint.com using the same email you used to set up the subscription.
Once you are on your account page, click on “Billing information” in the blue banner above your listed shipping address.
You will then click on the blue box that says “Update card” (this is also where you can update your billing address).
Enter the credit card information, to save click on the “Update card” button.