If the date and time for a booking needs to be changed, you should click the small wheel icon, which you find next to the Start Lesson button under My Lessons. This will then show you 2 options:
Cancel lesson and Reschedule Lesson.
You should only Cancel the lesson in case you never want to teach the student.
If you just want to change the date and time of an already booked lesson, you should click the Reschedule button and either immediately propose your studenty a new lesson time suggestion, which the student can then confirm and book, or write the student a message with a few time options for the future lesson. When the student writes back what time works for him/her you can then send the official Lesson time Suggestion.
The reschedule button makes the process much easier for both teacher and student, since they are already in the system, and in many cases the student has already made purchase, which stays in the system when you reschedule a lesson.