From 
Image 1 of 2
1. Unscrew the showerhead by gripping the shower neck with a pipe wrench (as shown), grabbing the nut on the showerhead with the adjustable pliers and turning the pliers counterclockwise until the nut loosens. Protect the finishes on the showerhead and wall pipe by wrapping them with a cloth.
Image 2 of 2
2. Open the showerhead holes by soaking the head overnight in a vinegar bath and poking the loosened mineral scale free with a toothpick. Rinse the showerhead in tap water, then reinstall it by applying Teflon tape to the wall pipe threads, screwing it on and tightening it by reversing the technique in Photo 1. Complete the repair by turning on the cold water in the shower and blasting out any remaining mineral gunk.
Why go to all that trouble??? I just hang a baggie (using a rubber band to hold it up around the pipe) filled with vinegar which covers the entire head and let it hang all night.