Flip the trimmer upside down with the power off to get a better angle at the head. Remove any grass, dirt or debris if need be before manually unscrewing the head in a clockwise direction. Once loose, pull the line spool free of the trimmer. Unwind any remaining line and pull it free from the tiny retaining hole on the inside of the spool.
Thread one end of a spool of new line into the retaining hole and wind the slack of the line around the spool in the direction indicated by the arrow on the head. Use about 3 feet of line total and snip the line away from the spool once done. If your particular model has the dual-line feature, wind the same amount of line in the opposite direction as the first line, following the given arrow.
Place the spool back into the head, pulling about 3 inches of new line away from the spool. Do the same for dual trimmers. Thread this small bit of length through the hole(s) on the head before screwing the head back onto the trimmer in a clockwise direction. Power up the trimmer to test out your work. If the line snags, repeat the steps and wind the line tighter to the spool; otherwise, pull additional line out from the spool as you work.