Login to your Content Sparks Learning Center account and look for the little icon or avatar in the upper right corner. If you click on that, there should be a drop-down menu with options for managing your billing and other things.
Select the one for editing your credit card. You might have to add a new one entirely if your old one expired.
After you add your new card, you'll need to look for Content Sparks as one of the 'Schools I'm enrolled in' and 'Select Card' over on the right to pick your new card.
And that should do it!