You can cancel any of your bookings 4 hours beforehand from "My Bookings" section .
You have 2 cancel right per week.
If you want to cancel your bookings less than 4 hours your minutes will be reducted from your account.
1) Go to "My Bookings" section
2) Drag related booking left from the right
3) Click "Delete" button
Your booking has cancelled.
Do not forget to book again for your next session!