From the home screen, click on the Schedule button.
Navigate to the applicable appointment and select the block.
Click the Edit Appointment dropdown and select whether they were a Show or No Show.
A Show will display a checkbox on the top right of the appointment block, while a No Show will show an X.
For repeated no-shows, you have the option to disable future bookings for the customer, which can be reversed from the student's profile under the Options tab.