You can cancel a customer's booking from the session Attendees screen. This is found at Schedule > click on session's Details button > Attendees > click on the customer's name > Cancel booking.
To move / transfer a customer's booking you cancel it following the steps above and then add them to a different session by going to Schedule > click on new session's Details button > Attendees > Add attendee.
You may also be interested in our Auto-refund product which enables you to auto-gift a credit when someone cancels in line with your cancellation policy, saving you the hassle of having to action the move / transfer.