Once you have a reservation created, you can edit it by first logging into the telescope that you have reserved via the Reservation Drop Down Menu.

This will bring you to the Make a Reservation calendar.  

From here, locate your reservation and click on it to edit it.  This will give you a pop up and allow you to adjust the start time, end time, and plan to run for the reservation.

Please keep in mind that there is currently a bug that will force the edit function to automatically select "No Plan Selected".  This will be repaired soon, but for now requires you select your plan each time you Edit your reservation.