Its only the black part that comes off,not the silver outer rim.wish someone had told the previous owner of my van this :x big chunks missing from the inner rim
the horn is held in place by a circular spring clip often rusted in place, gently work a blade screwdriver between it and the silver ring and just take your time leavering it out....
Whatever you use make sure it has a very fine tapered edge to get under the black plastic. I have found most screwdrivers to be just a bit too blunt. I use a one inch putty knife. I can work the edge under the plastic and I have a rather broad surface with which to lift/lever.