Turn the Weed Eater off and let it cool slightly. Set the tool on a flat surface with the head facing up.
Grasp the tap button firmly with one hand. Hold the Weed Eater shaft with the other hand. Pull the tap button straight off. Pull the line spool straight off.
Clean the spool and hub on the tool with a towel to remove all grass clippings and dust.
Stretch out a measuring tape on the ground to the 12 1/2-foot mark and lock it. Unroll the replacement string next to the measuring tape and cut two 12 1/2-foot pieces with a utility knife.
Insert each string into the small holes on the inside of the spool with ½ inch on the spool inside. Grasp each string and wind them together in the direction of the arrow on the spool. Wrap the strings evenly and tightly until 3 to 5 inches remain.
Push each of the strings through the exit holes in the hub on the Weed Eater head and turn the spool to align the exit holes and notches. Push the spool firmly down to lock it into place.
Pull each of the two strings firmly on the outside of the hub to release them from the locking notches.