If this was my hen I would clean the area with rubbing alcohol and lance it with a sterile scalpel, then I would dip a q-tip in hydrogen peroxide and clean it out. It looks like she scratched her head with her toe nail and allowed some infection to get in the wound.
Have some one hold/restrain her for you.
cut a small insision with the scalpel.
I would personally use an iodine solution (1 part iodine 3 parts water in a syringe with NO needle) squirt gently into the wound to sanitize and rinse out whatever is in there. Leave open to continue to drain.
Seperate your girl from the others and keep her in a very clean area and change litter frequently.
Jody is right, you only use hydro perox intially to kill germies. You can get scalpels at tractor supply. It will only hurt for a second, chickens are tough. It is important you get all the nasty pus out of it.
Betadine solution is good too. Does anyone worry about this being more toxic and being absorbed in the tissue,especially around the head??