A Recall that you have booked as an appointment shows an occupied appointment slot, and a "Booked" status in the Recalls Tab:
To cancel this booking, click to select it in the Appointment Book, and then click the Cancel button at lower right of the screen:
Click Yes to confirm in the confirmation screen:
Use the selector controls to add a Cancellation Reason, and then click OK:
Type Notes if necessary, and then click OK:
The appointment slot again becomes available, and the Recall reverts to its previous status in the Recalls Tab:
(Note how in this example the Recall has reverted from "Booked" (green) status back to "Failed" (red) status. This happens when an appointment date is missed and the appointment is not moved on. A Booked status only applies when it assists in the future)