Your ATV tires take a thrashing, it's just the nature of the sport, but when your tire comes off the rim or starts losing air while you're on a ride, you need to know how to properly seat your ATV tire so you don't get stranded.
Since ATV tires can be such a problem you should really be prepared by bringing some common tools with you. For this job you need: A small air compressor that has a quick release hose attachment, a webbed tie-down clamp and needle nose pliers.
To seat your ATV tire again you will need to take the rim completely off. You can try simply tilting your ATV if you have some help, but trails are notoriously uneven and if your ATV gives way, you could be seriously hurt. Better to take off the rim and play it safe.
Now using your webbed tie-down, wrap it all the way around the ATV tire and crimp it nice and tight. Next, remove the inside tire valve with needle nose pliers. Remember, "to the right is tight" so turn to the left. Using your air compressor, fill the tire with the quick release hose, and if you notice air is leaking anywhere around the rim, try tightening the crimp a little more.
Once you have the ATV tire properly seated again and no air seems to be leaking, quickly screw the valve back (righty-tighty) on the tire, and continue filling the ATV tire to the manufacturer recommended PSI.