Turn off water. Remove handle (but I'm thinking yours is already off). Remove packing nut (nut behind where the handle goes). Hopefully you will find a nub of the stem that you can grab with a wrench, if so, rotate stem counterclockwise several times, the white packing material should begin to come out towards you. If there is not enough stem to grasp, dig the old packing out with a screwdriver or similiar device. Having rotated the stem counterclockwise several times, pull stem towards you. It may be difficult if any packing remains in the valve body. If you can't grab on the stem, get a new hydrant, preferably the model 17.