To change your credit card, log in to Schedugram, then go to the "Profile/billing settings" page, click the "subscription" sub-menu item on the right hand side and then press the "change card".
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.