I have replaced portions of windows in previous units - the hardest part is getting the window out, specifically getting the outside flange butyl tape (between the flange and the side of the trailer) to let go. A plastic drywall mud blade and strong fingers works best for me.
For flanged windows, there are screws on the inside flange of the window to remove, then with the help of the wife I worked from the outside while she applied pressure on the frame on the inside pushing the window out.
The glass company will disassemble the window, measure for the new glass and can insert a temporary lexan pane while the new glass is sent out for tempering (required). The temporary lexan allows you to re-insert the window while the new pane is being tempered.
I used new butyl tape on the final install with the new tempered glass pane in place.
Hope this helps,
Brian