Up to 24 hours before the start of the session, there must be a way to reschedule it. I'll try and send you some screen shots so you can check for sure whether there is something wrong with your profile or not.

Gracias Marta. This question was mine (Tim) The problem is that when I go to the session to reschedule there is no "Change Date/Time" to select. I don't want to actually change our upcoming session but if I did there is no way for me to do so.

On your dashboard, go to 'Sessions' (column on the left or icons at the top right), you'll have your session there. Click on it and in the square with the session details click on the button 'Change date/time'. Select the date and time you want your session to be rescheduled to and send the request to your teacher.