Go to the travel center section and click change travel. You can choose to change one way or both ways. Click the one you are interested in and the dates and then you can choose from the options. Don't forget to go all the way through and click yes, I want to make the changes.
SWA does do not charge a fee to change flights, however you will have to pay any difference in fair (or get a credit if the fare has gone down).
So if you paid $100.00 r/t origianlly and now the lowest fare is $125.00 you will have to pay the $25.00 to make the change.
If the fare has gone down (and it does from time to time) and it is now $80.00, you will have a $20.00 credit on SWA. BTW, the credit must be used one year from the date the ticket was PURCHASED.