I did this on my 90 560sel, '89 & 93 300E with the info from these sites

http://www.mbclub.co.uk/forums/showthread.php?t=23963
and also http://www.peachparts.com/Wikka/Trans7226Fluid

From my memory, I had to take out the ( black plastic) dust cover and use big screw driver to turn the engine. It's not that hard. Good luck