You can add patients to the "Waiting List" by selecting the Waiting list tab on the Home page.
Select "Add Patient to Waiting List".
Set the criteria for the patient.
When you cancel an appointment on the Schedule you will see a Waiting List check box.
Select the check box. You will have an option to select a patient from the drop down list who matches the criteria of the appointment you are cancelling at which time you can swap out the patient name.
The patient will get an email reminder but keep in mind the patient has not confirmed they can attend so it would be a good practice to always call patients on the Waiting List to ensure they are able to show up so you don't hold a spot another patient could use.
You could also use the Waiting List as a Call List to contact the patient before booking their appointment.