Replacing string on your trimmer requires removing the head of the string trimmer. The string spool is often referred to as the head of the trimmer. Although there are slight variations in the actual removal process among the many manufacturers, the general process is relatively similar. Most common stringer head removal does not require the use of any hand tools. Most trimmer heads reassemble in the reverse order of removal.
Pull the spark plug wire away from the spark plug, if your trimmer is a gas model. Unplug the trimmer from the power cord, if you have an electric trimmer.
Place the trimmer on a work bench or on the ground, on its side so that you can access the string head.
Look for small plastic squeeze tabs on the side of the outer housing on most electric trimmers. Squeeze the tabs inward with your fingertips and pull the stringer head away from the trimmer.
Find the bump knob on the center bottom of the stringer head, if you have a gasoline powered trimmer.
Grasp the outside housing with one hand, and twist the bump knob counterclockwise with your free hand. Remove the bump knob, and pull the stringer head away from the housing with your fingertips.