You can easily update the bank account number from a customer that has an active subscription by removing the old IBAN number (mandate) and add the new one. There are 2 ways to add a mandate:
Via a payment link
Create a payment link for the customer and choose as type "Mandate". As amount you can enter for example €0,01.
As soon as the customer has paid the payment link, a new mandate has been added. From that point, the new IBAN number will be used for the subscriptions. (also for subscriptions via orders)
Manually add a bank account number
With the RCUR advanced plan (or higher) you can manually add a mandate without interaction of the customer. Go to the customer in your RCUR dashboard and klik on "Add bank account". To prevent confusion, you can also add the old mandate.