Edit 'Only this event' in recurring event series.

edit_recurring_event#1.rb

# Event schedule assumed here is: Weekly 5 times on Tuesdays, Wednesdays, and Thursdays
irb> existing_event.schedule.all_occurrences
=> [2013-08-20 16:09:08 +0530, 2013-08-21 16:09:08 +0530, 2013-08-22 16:09:08 +0530, 2013-08-27 16:09:08 +0530, 2013-08-28 16:09:08 +0530]

# Selected option - Only this event
# Event time being edited is 2013-08-27 16:09:08 +0530

# First, create another event on 2013-08-27 16:09:08 +0530 
# with updated params.

# Then add exception time to recurrence rule of existing event.
# Following are the commands.

irb> schedule = existing_event.schedule
=> Weekly 5 times on Tuesdays, Wednesdays, and Thursdays

irb> schedule.add_exception_time(Time.parse("2013-08-27 16:09:08 +0530"))
=> 2013-08-27 16:09:08 +0530

irb> existing_event.schedule = schedule
=> Weekly 5 times on Tuesdays, Wednesdays, and Thursdays / not on August 27, 2013

irb> existing_event.save
=> true

irb> existing_event.schedule.all_occurrences
=> [2013-08-20 16:09:08 +0530, 2013-08-21 16:09:08 +0530, 2013-08-22 16:09:08 +0530, 2013-08-28 16:09:08 +0530]