There are two ways that you can edit a clients appointment.
From the menu,
Click Calendar.
Select the appointment you would like to edit.
Click Edit Appointment.
Edit the details that deem relevant.
Click Save changes.
Alternatively, from the menu:
Click Clients.
Select the relevant client.
Tap Appointments and Classes.
Find the appointment you wish to edit.
Tap the 3 dots.
Click Edit.
Once you have succesfully edited the appointment, click Save changes.