post https://sandbox.billwerk.com/api/v1/contracts//changepaymentmethod
You can select whether the customer pays "On Account" or whether no payment data is stored for this contract by choosing "Without Payment Data" (selectedPaymentMethod : "None:None")
With the payment provider Payone, you also have the option of storing or changing the debit memo data for your customers.
If your customer wants to pay via Paypal, credit card or direct debit, you can make the self-service customer portal available to him, in which he can enter his payment data.
You can find an explanation of the customer portal here.