There will of course be occasions when you will need to cancel a lesson due to sickness or unexpected circumstances. To cancel a lesson, log in to your account and click on the "Lessons" tab on the left. You will see your list of lessons, click "cancel" on the relevant lesson and you will be prompted to explain the reason for your cancellation, which will be sent to your client.
You should also send a message to your client through the Messages section, and perhaps work out a time that you could rearrange.
By default, if you cancel a lesson, that lesson time will be removed from your availability (assuming you can no longer make that time). If for some reason (e.g. a tube strike prevents you from reaching a particular location) the lesson can be re-sold to another client, you can elect to add the lesson back into your availability calendar.