Disconnect the spark plug wire from the spark plug.
Turn the string trimmer upside down, with the trimmer head facing upwards. Remove any grass, weeds or other objects that may be wound around the trimmer head.
Hold the trimmer head securely with one hand, and rotate the knob in the center of the spool counterclockwise until the spool stops turning. Then, pull the spool out of the trimmer head.
Remove any loose segments of string from the spool.
Insert the end of the new string on the replacement reel into the hole in the center of one of the channels inside the spool. Wind the replacement string around the spool in the direction shown by the arrow on the side of the spool until the channel is filled. Slip the string into the notch on the side of the spool to hold the string in place, then cut the string about 6 inches away from the spool with a pair of scissors or utility knife to separate the newly spooled string from the replacement reel.
Repeat the previous step with the second channel in the spool.
Place the spool over the trimmer head, and insert the two ends of the string through the holes on opposite sides of the head. Nestle the spool into the head while pulling the exposed string through the two holes. Hold the head firmly with one hand, and push the spool into the head, then rotate it clockwise until it locks into position.
Turn the trimmer over and connect the spark plug wire to the spark plug. Start the trimmer and test to be sure that the unit is functioning properly.