You got a hitch? Lay it on the ground under it and use a jack to break it. You do it from the road side of the wheel. Remove the valve core first to let all the air out and help it vent better. Use a 2x4 to hold/wedge the tire bead open and use a knife to cut the old valve stem. Lube up the new one and pull it through. You will need a valve tool that threads onto the stem so you can get enough pulling/prying force on it to pop it into place. And don't drop anything in the tire in the process. It's a skills test but I've done it a few times.