Turn off the trimmer and lay it down with the head facing up.
Hold the outer portion of the spool still and turn the bump knob counterclockwise to unscrew it. Remove the bump knob to expose the inner reel.
Pull the old inner reel out of the outer spool and remove the spring from the bottom.
Transfer the spring to the new, pre-wound inner reel.
Insert the free ends of the string wound around the new inner reel through the eyelets on the outer spool.
Place the inner reel into the outer spool and push the two components together to lock them in place.
Grasp both pieces of the line and pull it firmly to release it from the holding slots.
Insert the inner reel and outer spool back into the trimmer head, then thread the bump knob onto the head, turning it clockwise.