Lots of other posts here about changing flights, but here's the summary. If you view your upcoming trips online you'll see a "change" option. When you select that you'll be asked which flights in the trip you'd like to change. In this case it sounds like you would want to change both the outbound and return flights. Then search for and select new flights and you'll be informed if there is an additional balance due or if the new flights are cheaper you'll be issued travel funds which you can use for future flights. You can also do all this over the phone if you'd prefer.
Give it a try and reply how it goes or if you have additional questions.
