Follow these steps in order to easily add or update your Credit Card:
First, head over to your 'Billing' page on the left menu, under 'My Settings':
Then click on the 'Manage Subscription' button:
You will now see your current Credit Card (if you have one already). Click on the active Credit Card if you want to update it, or click on the '+ New Card' button to add a new one.
Click 'Save' when finished and your new Credit Card will get added.