When you go to "Settings" and then "Appointment Reminders", make sure to add the "Cancellation link" to the email body, along with any other information you may want:
When a patient books an appointment, they receive an email like this:
The incoming email account for online bookings receives an email like this:
If the patient clicks the red hyperlink "Cancel booking" in their booking confirmation email as above, they are taken to this page:
When they click the red "Cancel my booking" button they see the below page, with the option to click "Yes" or "No":
If the patient clicks "Yes", they are then redirected to any page you like, perhaps your homepage (you choose, for example you might choose to redirect them back to your booking page).
You need to enter your URL of choice in the "General Settings" section in the "Online booking" settings tab, as circled below:
When the patient cancels their booking, the incoming online booking email account receives an email containing this: