to flush out all the ATF, disconnect the return line from the cooler and guide into a bucket, start the engine and drain 2 litres and stop the engine, now add 2 litres to the transmission, start again and collect 2 litres and stop the engine, refill with 2 fresh litres. start again and collect another 2 litres and stop the engine. connect the cooler line back and refill the last 2 litres.
By the last 2 litres - you will see fresh ATF from the line (use clear line or a white container to see the colour of it)
You have now changed about 95% of the ATF in there - the rest 5% is stuck in the clutch packs.