Water/coolant in transmission fluid, what now?

If you've never replaced your engine's radiator, it's not an inexpensive repair. Then simply have your transmission flushed and new fluid installed. To do the best job, drop the pan and replace the transmission filter as well. Fear not, you've diagnosed the problem and now will fix it! One question is whether the radiator coolant pressure is high enough to force itself through the transmission heat exchanger into the transmission. But you say it happened, so we'll have to go on that basis.