You can change your payment details in the app via Profile at Change payment method. The standard payment method is set on iDEAL. If you want to add or remove a credit card, you can do so here. If you remove an active credit card, you will be automatically transferred to iDEAL.
Please note: if you remove an active credit card and have not yet driven more than €75, you will first have to top up your credit with at least €75 before you can make a next reservation.