First thing to do is move the tractor so that the valve stem is at the top of rim, that way you wont loose all that fluid. Also, if you have a jack to use, take the weight off of the tire so it wont push anymore out. If its leaking out of the valve stem then a new valve should fix it. If its leaking around the valve steam then the tire is most likely gonna have to come off.
O, if the tire is allready sitting on the rim then dont try to drive the tractor to put stem at top as it will probably unset the tire