Set the brush cutter on a flat workbench. Align the hole in the flange washer, located directly behind the blade, with the hole in the gear head. Slide the head-locking tool through both holes. Place the 1/2-inch socket on the nut holding the blade to the gear head and turn clockwise to remove.
Slide off the nut, washer, blade and flange washer. Remove the three screws holding the brush cutter guard against the gear head. Remove the guard.
Install the grass deflector. Slide the metal mounting bracket on the top of the drive shaft just behind the gear head. Place the grass deflector under the bracket. Insert the tab on the bracket into the slot on the grass deflector. Insert the screw that came with the kit through the hole in the bracket into the hole in the deflector and tighten.
Slide the flange washer onto the gear head that was removed in step 2. Place the head-locking tool through the hole on the flange washer and the hole in the gear head. Turn the new trimmer head counterclockwise until tight against the flange washer.