See if you can break the bead and clean it up. Break one side of the bead at a time. Then, once you clean both bead seating surfaces on the rim real good, use either a good compressor or a CO2 tank to re-mount the tire. You want to use a bunch of pressure to pop that bead into place. I'm not sure of the correct amount to use on an ATV tire.....but usually you use a lot more than normal inflation pressure. If that doesn't work, then have a professional re-mount it for you.
If that STILL doesn't work, maybe you need beadlocks. If you can't afford real beadlocks, there are redneck ways to keep the beads from spinning on the wheel. Screws behind the bead and into the bead to hold them in place. It requires more attention to air pressure, because it won't hold air quite as well.