I have an electric trimmer or weed whacker and am not quite sure how to replace the line, does anyone know how to?

Loosen the trimmer head mounting bolt or knob. Remove said head. Take about fifteen feet of line, and if it is a two string trimmer, bend it to make it a "U" shape and leave one end about six inches longer than the other. Now insert the line through the head's hole or knotch. Begin winding in the direction shown on the spool. Keep the line taut and moving from one side of the spool to the other. When at the end, place the two ends into the knotches on the outer edge of the spool and insert the line through the holes. Pull both lines at the same time and then replace the bolt or knob.