Start by loosening the bolt then tapping the stem down in the normal way. Next remove the bolt entirely, and try twisting and lifting the stem at the same time, If the bolt is wedged against the internal swage it might resist, so set a wooden block against the bottom of where the extension meets the quill and give it a sharp upward rap with a hammer. That should pop the stem free, and you can retrieve the expander later with the bolt, or by removing the wheel and pushing it out from the bottom.