You can cancel an appointment made by your attendee, you have a few options:
1. Access your Google Calendar, Office 365, or Outlook Calendar to reschedule or cancel the appointment directly.
2. Your attendee can utilize the options provided in the confirmation email to reschedule or cancel the appointment.
3. If the appointment was booked via email, your attendee can reschedule or cancel directly from the original email.
And if they wish to reschedule an appointment, you can find the steps here.