This is an easy one! Log in as an Administrator. Go to Settings > Billing Info.
Here you'll be able to update your credit card on file, as well as your billing contact. When changing the card on file, you want to make sure your billing address matches the one associated with the new card you're adding! It's one of the biggest reasons payments don't go through.
Lastly, hit the trusty olebutton. And you're done! =)