Open it
Schedule → Appointments, or select it on Schedule → Calendar.
Reschedule
Choose a new time. The customer is notified of the change automatically when notifications are on.
Cancel
Cancelling frees the slot immediately and notifies the customer. Anyone waiting for that time on your waitlist is told it has opened up.
Mark what happened
Update the appointment's status so your reports and your client history stay honest — including no-shows.
Notes
Add notes to an appointment for anything worth remembering next time. Notes are internal and never shown to the customer.