Yes, you can manually move a customer to an hour as an administrator. This also works in retrospect.
The shifting resp. Rebooking a customer consists of first canceling the booking or logging out of the customer and then having to rebook or sign up.
You can do this in two ways. Either you can change this via the customer details or the participant list of the hour.
First go to "My Customers" and then select "See Customer Details" at the customer you want. At the bottom you will find the section "Bookings" where you can cancel the booking via "Action" and "Cancel". After doing this, you can book a new lesson for the customer.
Alternatively, you can search for the hour under "Timetable bookings" and select "View participant list". Here you can unsubscribe the participant and add a lesson again via "Add participant".
Read the article "Manage participant list / bookings" for more information.