Teachers should set availability for when they know they can take students. You should not regularly unschedule or cancel lessons because of conflicts in your schedule.
If your schedule is variable, you can change your availability for each week by following the instructions here.
You should unschedule or reschedule a lesson with a student only in extreme situations when you cannot possibly attend the lesson due to a change in your schedule. You must communicate the issue to a student, and inform them that they can reschedule the lesson at their convenience.
To learn how to unschedule or reschedule a lesson with a student, please, see this article: https://support.verbling.com/en/articles/2299133-how-do-i-reschedule-a-lesson-with-a-student