Unplug the spark plug wire from the spark plug on your Troy-Bilt trimmer. Grab the spark plug wire where it covers the spark plug with your fingers and pull it straight off the plug. Place the trimmer on its side with the fuel tank filler neck pointing up.
Insert the end of the locking tool in the hole on the head between the blade and the gear housing. If you do not have the original locking tool, insert a finishing nail into the hole. Rotate the blade by hand until the inner gear housing hole lines up with the outer hole. The tool or nail will slip into the inner housing hole.
Hold the nail or locking tool inside the hole and turn the securing nut in the center of the blade clockwise with a socket wrench. Remove the nut and the blade retaining washer. Pull the old blade off the trimmer shaft.
Place the new blade over the trimmer shaft. Place the retaining washer over the shaft and thread the retaining nut onto the shaft. Hold the locking rod or nail in both housing holes and turn the nut counterclockwise to secure the blade.
Remove the nail or locking rod from the hole and press the end of the spark plug wire over the spark plug.