Most of the fluid is in the torque convertor and valve body - you cannot just drain it from there unfortunately.
Some garages have machines to replace it all, but power flush machines should be avoided.
To DIY you can disconnect the hose going to the cooler, start the engine, let 1 l come out, stop the engine, add in 1 l and repeat until the fluid coming out is a nice bright red.
It is probably safest to just drain and refill the pan three of four times, once a day/week maybe.