How can I trade in a car I'm making payments on
It all depends on if you are upside down on the car. If you have equity then you won't have any problems. If you owe money you will have to pay it off or have the balance rolled into the next car loan which is a BAD idea as your payments will be even higher and you will be even more UPSIDE DOWN.
And you'll be paying for two vehicles. One of which you no longer have. HTH. -Jim