under Tools on southwest.com choose
Change Air Fare
once you put in your confirmation number and name ( bought under ), it will guide you through the process
if there is a credit you will get a Ticketless Travel credit which can be used for future travel for one year from the date you booked it ...in this case May, so it would be good until next May ( date of previous booking )
right now those funds can be used by anyone to help pay for a booking, but in Frebruary 2010 they are going to restrict bookings with those Ticketless fees to the person whose name was on the original booking