You need to make sure that you have created a new available schedule of the time you are moving the appointment to, for example in this case, we have added a schedule for March 12 at 11:00AM:
2. From the booking, click on the "edit" button against the booking:
3. Click on the "change time of this booking" button:
4. Where the system would allow you to choose an available time on the calendar page:
5. When you have chosen a time and date, the system would show the message that you have successfully moved the customer to the updated time and date:
***If you need to move an entire class or multiple customers at once, kindly see link below for reference:
https://help.flexbooker.com/en/articles/2703568-how-to-update-a-booking-time-and-date-for-an-entire-class-or-multiple-customers-at-one-time-using-a-fixed-time-schedule