The easiest way is to go t=o the reservation overview on your launchpad and use the delete (bin) function
You can also cancel a reservation by following the below directions:
1) First, we need to log on to the telescope where you made the reservation. The easiest way to do this is to use the Reservations drop down menu on the Launchpad as pictured below. In this case we will select Telescope 11.
2) Once selected, the Launchpad will prompt you to enter your Username and Password and you will find yourself on the "Make a Reservation" screen pictured below. Use the calendars on the left to choose the date the reservation you wish to cancel is set to begin, and locate your reservation (hint* Your reservations will appear in a gold box, other members in green)
3) Once you click on the reservation you wish to cancel you will see the image below. Simply click the "Delete" button that is circled in the image and you reservation will be cancelled. Keep in mind that if you have "Show My Deleted and Past Events" checked, cancelled reservations will have a red bar on the left side.
For more information on the Reservation System, check out the Video Tutorial: