From the confirmation or reminder email, click the 'view appointment' button at the bottom.
Click the cancel button to cancel the current appointment and schedule another appointment. All calendars will be available to you at that point. You will have to confirm that you wish to cancel. The next screen will start the process to set another appointment on a calendar that has the most convenient time for you.
Note - the 'reschedule' button would only give you the availability of the person whose calendar you were currently on. In the example below, it is on Tammy's calendar and would only show Tammy's availability.