You can change southwest airlines flight tickets through your southwest account online.
Once again, note that changing your online flight will make a difference for all travelers. If you want to change the trip for one person, you should call Southwest.
To change southwest airlines flight, you must click the “Change flight” link on your flight.
Your original flight itinerary will be filled in and you can choose new dates or an entirely different destination. Or, if you’re looking to re-evaluate your flight, you’ll want to save the information as it is and click the “Browse Options” button.
When you see new flight options, it’ll immediately show you how much more or more each option costs than what you originally paid. Southwest will show the difference in dollars or points, depending on how you paid.

You can opt for cancellation if you don't want the change but before this follow the Southwest airlines cancellation policy