To change your Credit Card/Debit Card information linked to your gym, you will need to log into your account at www.btwb.com. Once logged in, click on your gym drop-down menu in the top-right corner of the header, then again on "Admin Console".
From your Admin Console you will need to select Settingo n the left tab >> Subscription
Select Manage Subscription
You can then select the Pen icon to edit the card next to the specific subscription.