Trading my new car for a newer car?

i had just gotten a scion xb 2006 in october and i want to trade it in for the new scion xb 2008.when I purchased my car it was 16,851 because of stuff i added to the car. The new scion is around 16,500. I have only paid off 2,000 on the car. what would i need to do to get the new scion.