Without a picture, it's hard to be sure, but it sounds like you have an A&E/Carefree awning. If so, there should be a lever at the front part of the roller tube. You would roll the old awning out, make sure the lever is engaged (it is a brake), and take the other end of the roller off the tube. The awning will then slide off the tube in that direction.
You will also have to slide the awning off the awning rail of the trailer. It should have a set screw holding it in place. Remove the set screw, and slide the awning off the rail, like you just did with the tube.