How do you replace troy-bilt string trimmer head?

Looking between the gear box assembly (metal housing) and the top of the trimmer head, locate a u-shaped slot found on either side of the gear box assembly. If you rotate the trimmer head slowly while looking into that slot you will see a mating u-shaped cutout. Place a screwdriver, Allen wrench or anything that will prevent the internal mechanics from rotating. Now turn the trimmer head counterclockwise and it will unscrew from the gear box shaft.