You can edit a rating by going to your booking page. If you are also an instructor, make sure that you click the customer toggle to see all your bookings as a customer. Then click the booking whose rating you want to edit. Under the feedback section, click the edit button to change your rating.
Note that you have 14 days after the end of a booking to edit your rating. After this rating period has passed, you can no longer edit it.
The instructor will receive a notification when you edit your rating.
If you want to learn more about creating a rating, then please read this article.