Replacing a French door involves detaching the old door from the frame, cleaning the doorway and attaching the new door onto the frame with hinges. If needed, replace the frame with a new one before attaching the new door, and caulk any openings around the door.
Start by gathering a screwdriver, a hammer, a wet rag, a carpenter's level and a cleaning solution. Add shims, silicone caulk and two-by-four wood if you intend to replace the door frame. Once the necessary tools and materials are in place, use the hammer and screwdriver to tap away the hinge pins. Unscrew the doorjamb and threshold, and detach them from the door.
To replace the door frame, first pry the old frame from the place, scrape away any old caulk from the doorway, and wipe the doorway of any dirt and caulk remnants with the wet rag. Cut and assemble the two-by-four into a new frame according to the required measurements, set the frame in place with shims, adjust the frame to make it level using the carpenter's level, nail the frame onto the doorway, and cut off excess trims.
To attach the new door to the frame, first verify that the threshold is level with the carpenter's level. Set the door on the frame with the level, and attach hinges. Finally, seal the openings with the caulk, and apply paint over the caulk. Allow the paint to dry, and attach any door hardware to complete the process.