In terms of changing the ATF, Andrew is right with the procedure, but there is another caveat and that is how long is it since it was changed. There is a considerable amount of ATF in the system that is trapped in there and only comes out if you take it all to bits. If you regularly drop 5 litres and put in 5 litres you are essentially continually diluting the dirt. If it hasn't been changed for years and it comes out black then you need to drop and swap, drive for a bit then drop and swap again until you have diluted the dirty stuff to an acceptable point. I put 20 litres though my last truck until is was clear.
Chris