Yes, a client/consultant can reschedule/unschedule a session till allowed duration(set by Admin). To reschedule a session:

Sign in to the account. Go to "Session ".
The scheduled session will be listed under the "Session ".
Click on the "Reschedule Session" icon of the missed session .
A pop-up box will open. Mention a comment for rescheduling the session for the client and click "Send".