keep holding the button in and pull the head rests all the way out. These are the worst designed headrests ever. They make my head have to tilt forward unless I lean the seat way back. So I took them out and put them in backwards.
I can't remember if this was on my Expy or another vehicle, but I do remember having to press in a tab that normally sits flush on the "other" post to remove the headrest. You may want to give that a shot.