When your client books an appointment the receive a confirmation email. In the confirmation email is a option to View/Cancel their appointment. Your client can click the link in the email which will take them to a cancellation page. As long as the timeframe is prior to your selected cancellation notice requirements the client can cancel the appointment online.
Your client can also cancel directly with you, all you have to do is open up the appointment and click on the delete button. This will remove the appointment from your calendar, cancelling it for the client.