You can cancel your booking before it starts, according to a specific timetable (see next question).
With regard to changing a booking, you can postpone your return time, provided that the extension does not conflict with a subsequent booking you have already made for the same vehicle. If, on the other hand, you return earlier than planned, you can bring forward your return time by entering the actual return time via the app and, if applicable, you will be entitled to a refund of 50% of the unused time. If no changes are made via the application, no refund can be applied.