To replace a trailer window, pry out the old or broken window, measure the window, remove old putty from the window frame, apply new putty, and install a new window. Open and close the window to test the installation. Required tools include a drill, screwdriver and square.
Remove the siding, remove the old window and frame with a drill, and set aside them aside. Discard the old screws. Measure the window's height and width, and use the measurements to purchase a replacement window. Scrape old putty from the window frame with a wire brush.
Apply a continuous layer of new putty on the window frame. Be sure to apply at least two layers of putty at the top of the opening to resist moisture. Loosely fix the frame in place, making sure to only attach one screw at the center on all sides of the frame. Ensure that you use new screws of the same type as the old ones.
Open and close the window to ensure it is opening freely. Check all corners with a square. Attach the remaining screws, and then fasten them with a screwdriver. Apply sealant along the edges of the window, and reinstall the siding.