The trimmer head on your weed trimmer consists of the trimmer line spool and outer housing. When replacing the trimmer head, the drive shaft on the inside of the trimmer shaft requires locking. The trimmer head attaches to the drive shaft with a nut. Locking the drive shaft enables you to loosen the nut and pull the outer housing of the trimmer head away from the trimmer. Replacement trimmer heads are available at home improvement centers and come with line already on the spool.
Unplug the trimmer power cord from the extension cord, if you have an electric trimmer. Gas model trimmers require removing the spark plug wire from the spark plug. This prevents accidental starting of the trimmer while you are working on the trimmer head.
Grasp the outside of the trimmer head with one hand and press in on the line spool with your free hand. Twist the line spool left or right to disengage the spool from the outer housing. Some trimmer heads have a bump feed button in the center of the line spool. Twist the bump button counterclockwise with your fingers and remove the bump button and line spool out of the housing.
Look on the end of the shaft above the shield for a hole. Rotate the outer trimmer head housing with your hand until the hole on the outside of the shaft lines up with the inner drive shaft hole. Insert a small metal dowel or a small Phillips-head screwdriver into the hole. This will prevent the drive shaft from rotating while you remove the rest of the trimmer head.
Remove the nut on the inside center of the trimmer head housing that secures the housing to the trimmer drive shaft with a socket wrench. Pull the trimmer head housing away from the trimmer.
Place the new trimmer head housing over the drive shaft and secure with the retaining washer and nut. Remove the screwdriver or metal dowel out of the trimmer shaft. Insert the pre-wound line spool into the trimmer head housing according to the manufacturer's instructions for your model. Most spools have a spring on the upper center of the spool that fits over the housing retaining nut. Twist the spool left or right to engage the locking tabs. Plug the spark plug wire back onto the spark plug or connect the trimmer plug to the power cord.