In Sessions ("Training" > "Sessions" on the sidebar), Click "Cancel Session" on the session you want to rebook. You will then be asked to enter a date/time when you were notified of the change and why. Do so, leave the "Chargeable" Box unticked and click "Cancel Session". You will then be able to rebook the session by clicking "Rebook Session" and adding the new Start and End Dates/Times.