You'll need to add an unavailability from the Add a Schedule screen. To get there, just hit the Home tab, and select Staff in the submenu. Choose the staff member whose schedule you want to edit, and then click Appointment Availability.
- Click Add New Schedule.
- In Step 1, select Unavailable and enter a reason for the unavailability (e.g., Vacation, Sick leave, etc.)
- Enter the other details of the unavailability, and click Add. This will override any existing availability.