Go to your instructor's schedule where you built your account
Click in the upper right where it says "Login"
Once logged in you'll see a small circle with either your profile image or the first letter of your name. Click on that profile image and choose "Purchases"
Click on the 3 small dots to next to any active purchase or subscription
Enter new credit card information and save!
If there is no option to Modify, the card on file can be adjusted at the next purchase.