You can cancel a lesson at any time prior to a lesson starting, though you must make every effort to inform your student about the reasons for your cancellation. To cancel a session, just log in to your Tutorful account and go to your "Lessons" tab. Click on the lesson in questions and then click on 'Cancel Lesson':
If you have weekly sessions, you can either cancel just one session (if you know you'll be away on holiday, for example) or all of the sessions after a certain point (if you know that your sessions will end after that date).
If you just want to cancel just one lesson, leave the box asking if you would like to cancel this and future lessons unticked. If you would like to cancel all lessons after this one also, then tick the box.