Yes, a learner/mentor can reschedule/unschedule a lesson till allowed duration(set by Admin). To reschedule a lesson:

Sign in to the account. Go to "Lessons".
The scheduled lesson will be listed under the "Lessons".
Click on the "Reschedule Lesson" icon of the missed lesson.
A pop-up box will open. Mention a comment for rescheduling the lesson for the student and click "Send".