To rebook/move a booked meeting always use MyStore (and not Microsoft Outlook) for this;
Right-click on the work order (the blue box)
Then click on the Move to option
A new pop-up menu will show:
Under Resource, Start date and Start time you are able to change both the resource and time slot of the booking
After 60 seconds the work order and the appointment should have moved to the new time slot.
An email confirmation regarding the change of the booking will be sent automatically to the customer.