From the calendar views, click on the schedule you want to change to bring up the 'Modify Schedule' window. If you are using one of the list views, click on the 'Modify/Delete' link. From there you can either modify the schedule and save the changes, ignore changes you have made, or delete the schedule.
You will only be able to edit or delete your own schedules unless you have the ability to use 'Dispatcher Mode'.
If the schedule is already completed, you will not be able to cancel the schedule. If the schedule has begun, the portion of the schedule that is in the past will remain.