This depends first and foremost on whether or not your pupil has messaging enabled on their account. You can read more about this here.
Messaging not enabled:
If your pupil does not have messaging enabled and you cannot make a session, then there is no capacity for rescheduling this session - you should organise cover for the lesson instead. Read about how to organise cover here.
If your pupil does not have messaging enabled but lets you know in the lesson space during a lesson that they would like to reschedule a lesson with you, drop us an email in to firstname.lastname@example.org letting us know and ask the pupil to ask their teacher about rescheduling the lesson.
If your pupil does have messaging enabled and you would like to reschedule a session, send them a message asking them for another time they would like to rearrange the lesson for. Always drop us an email to email@example.com letting us know so that we can cancel the originally planned session. Be sure to email us again once you and the pupil agree on a new time to reschedule the session for so that we can book the new lesson in for you.