Busuu Live Lessons can only be cancelled, not rescheduled. If you cannot attend a lesson, you can simply find the lesson on your Live Lesson Dashboard under "Upcoming" and click on the three dots > "Cancel Live lesson".
Teachers require a minimum of 24 hours notice for lesson cancellations. If you cancel 24 or more hours before your scheduled lesson, the lesson will be returned to your "Remaining" lessons on your Busuu Live lesson dashboard. You will also need to cancel in the same month you booked the lesson to receive the credit back. Then, you can book a new lesson for your desired time.
If you cancel less than 24 hours before your scheduled lesson, this is considered a late cancellation and you will lose the lesson. If you do not cancel in the same month as you booked the lesson, you will lose the lesson. This is so that you do not exceed your monthly allowance of lessons.
For more, see How do I cancel a Live Lesson? (Student)