With every email that goes out from checkAppointments you can embed the email tag %APPT_CANCEL_URL% in the email which, when clicked on, will link the client through to a Manage Your Appointment page.
To make sure this link is on the email templates you want it on, read our documentation on Customizing Email Templates.
Once the client receives the email confirmation of the appointment or the reminder email x number of hours before the appointment, they will find a link to manage the appointment at the bottom of the email:
Please note that the email tag %APPT_CANCEL_URL% is what we are currently (as of 12/9/2013) using to link to the manage your appointment page. It will reformat to the appropriate b00k.us url for the appointment. We will update this with a new tag that is more appropriate for the action of managing the appointment but have yet to release it. We will update this documentation as soon as we do.
When they click through that link they'll get to a Manage Your Appointment page formatted the same way as your mini website. On that page they can take the action to reschedule or cancel the appointment:
The tag to allow clients to do this is on your email templates by default. Clients will not be able to cancel an appointment within whatever time frame you've set to allow cancel hours for.