I'll quote this from another site:
"You have to depress the spring loaded "tooth" that protrudes from the support post which is designed to prevent the headrest and post from just sliding out.
I'll try to explain - Run your hand along the backside of the seat so as to actually feel the support post behind the upholstery. Start at the top and try to feel along the sides of the post for a protruding barb or tooth. It's down about six or seven inches. Squeeze that in and the headrest post will slide right out. If you have to use pliers or a screwdriver, put a rag between the tool and the upholstery.
The explanation is more difficult than actually doing it. Once you find the barb, it will all make sense. Once it's out, use a little oil to
keep the spring steel easier to depress next time. Actually, once you see it, you'll figure out how to lock in place so you won't have to go through this every time you want to remove the headrest."
I hope this helps.
SC