A session can be cancelled and refunded up to 24h before it's scheduled. Your mentor will have a 24 hour period to dispute that refund – for example if the session had indeed already happened. If all is OK, the refund is being sent to you after that period.
To cancel a session, follow the booking confirmation link in your email.
In the booking overview, you have the mentor's contact information, scheduling information, ability to complete the booking and also to cancel it.
A cancellation is non-reversible. A session that has already been marked as "complete" by yourself or the mentor can no longer be cancelled.