You go online and rebook. It's very easy. Go in through your existing reservation and choose to "change your reservation."
I always recommend that people book 2 one way flights and not a round trip. Because one leg of the flight may have gone down and the other may go up, so if you try to change it, you won't be saving anything. If you book 2 one way flights and the flight out goes down but the flight back stays the same or goes up, you can rebook just your flight down and bank the credit. If it's purchased as a roundtrip, you can't rebook just one leg of the flight.
Just remember, you must use the credit within 1 year (booked and traveled) of when you originally booked the flight.