If you booked an Anytime fare or a Business Select fare you can get a refund of the price difference.
If you booked a Wanna Get Away fare you can get a credit (in the form of "Travel Funds") that is valid for travel within one year of the original purchase date. To get the credit, click on the "Change Flight" option on Southwest's home page. Enter the confirmation number for the flight you want to re-price. Pick the same flight as your "new flight". Make a note of your confirmation number. You will need that down the road when you want to apply the Travel Funds for another flight.