Trimmer lines becomes worn with each use. This nylon string can melt or be broken off by excessive wear against hard objects other than weeds and grass. Paying careful attention to the location of the trimmer head and removing large items before use will help extend the life of the trimmer line. When it is time, the string will no longer advance, signaling that replacement is necessary.
Remove the spark plug wire from the spark plug to prevent the trimmer from starting.
Twist the trimmer head counterclockwise while gripping the trimmer base to remove the head. Discard any leftover line that is still attached inside the trimmer head.
Cut a 9-foot section of new trimmer line. Fold the ends of the line to make a "J" and loop the "J" into the holes on the head. Wrap the line around the trimmer head in the direction printed on the side.
Pull each end of the trimmer line into the notches in the trimmer head to secure it in place.
Thread the line ends through the holes on the trimmer base. Reattach the head to the base, twisting in a clockwise motion until it snaps, signaling it is securely attached.
Replace the spark plug wire onto the spark plug.