Before replacing trimmer line, prepare the line by cutting between 10 and 25 feet off its length before installing it. Trimmers and weed eaters that have use two spools of lines require that lengths be cut off each spool. Pay close attention to any marks or arrows on the trimmer, as this indicates how to wind the line. To properly install the line, wind the string from side to side before starting overlapping layers.
To replace the trimmer line, open the trimmer or weed eater head by unlatching or unscrewing the top. Inspect the trimmer head, and look for either tabs or screws that release the retaining ring. After releasing the retaining ring, lift the old spool from the trimmer head and dispose of it. Replace the old line spool with the spool of new line trimmer. To secure the line to the trimmer, run an end of the string through the hole in the center of the spool.
Once the new line has been wound, use wire cutters or sturdy scissors to clip the notch on the edge of the new spool. Run the other end of line through the eyelets of the trimmer. Replace and screw in the retaining ring to secure the spool within the trimmer head. Replace the top of the trimmer head, and fasten it to the trimmer.