YouTube will help. If it is a non-ducted rooftop AC it's not much more than removing four bolts and undoing the electrical connection after you take the trim piece off the bottom. Before putting on the new one you'll use a new gasket and tighten everything down and hook the wires back up and you're done.
1992 D250 Cummins 5psd
2005 Forest River T26 Toy Hauler