You can't really. Classes are considered "locked-in" 24 hours before the start time. My only suggestion is that you *immediately* contact your teacher by *all available* means of communication to let him or her know you cannot be there. If you ask them nicely, they may or may not agree to let you reschedule the lesson.

You can't cancel a class within 24-hours of the start time, but you can message the teacher, and if they agree you can reschedule the class after the original class time passes. Make sure to message your teacher before the lesson though.