To change your credit card, log in to Schedugram, then go to the "Manage account / billing" page (accessible under the top right menu where your name is) and press the "change card" button.
You will need to be the administrator, or the person who originally set up the account. Invited users ('collaborators') can't see any billing information, or update it.
It's as easy as that! You'll be redirected to a separate secure page to update the card.