I do not think there is any such plug (on any model tractor.) The simplest and I think best is just to pull the thermostat housing , remove the thermostat, and wrap a rag or handy innovation around a garden hose and reverse flush the entire cooling system. You can direct the loose end of the top radiator hose where you need to for disposing of mildly contaminated water. I always use a bottle of Prestone Radiator Flush chemical in the cooling system for an hour or two of operation before doing this. Cannot claim this really does much but it seems the right thing to do. Afterwards you still need to get enough residual water out to make sure you are getting a 50/50 mix overall when you are done adding antifreeze. That's one reason I prefer to refill with full strength antifreeze rather than premix. Check your manual for what the total cooling system capacity is so you know how much antifreeze is enough.