Pete, you drain the fluid from the pan using the drain plug and also on this car you drain the torque converter. You may be better off having a MB specialist do this for you IMO.
Then you remove the transmission pan entirely and replace the filter, which is held on with 3 machine screws. After everything is put back together, about 3 quarts of transmission fluid (DexronIII/Mercon) is added through the transmission dipstick tube (use an appropriate funnel), then the balance can be added after starting the engine. It will need a total of roughly 6.5 quarts.