You most certainly can, however it does require a partner. Back before there were machines to do transmission flushes this is how we used to do it. Jack the vehicle up slightly depending on vehicle type. Disconnect the transmission fluid feed line from either the tranny cooler or the tranny itself (has to be the feed line) now with the feed line inside of a good sized bucket have a friend with a funnel ready to begin pouring new tranny fluid into the filler. Start the car and as the old tranny fluid exits the feed line in the bucket your buddy should be pouring new tranny fluid into the filler as soon as you see new tranny fluid begin to exit the feed line turn the engine off. Now doing it yourself will require a few additional quarts of trans fluid, but it is a small price to pay for saving on the labor. Good Luck!