Tap on the menu in the top right and choose the "Billing" menu option. If you don't see this option it means you are not the account owner. Only the account owner can access and modify billing information. If you think you should be the account owner, please contact us.
Once you're on the billing page, tap on the pencil icon next to the existing payment method. This will open a view were you can remove the card currently on file or add a new credit card.