Currently, it is not possible to modify a confirmed (paid) booking. If you are unable to conduct the requested product, you are encouraged to message the customer via Scubago and try to find an alternative product or date. You can do so via the messaging feature on the left side of the booking window.
After agreeing to a new product or date, you can cancel the current booking via the "cancel" booking, while the customer creates a new booking request for another product/date:
When you cancel a paid booking, the customer will get his deposit refunded and the current booking will be cancelled.