This procedure is from www.mercedesshop.com:

FSS reset procedure

It should be in your owner's manual. On my 1998 E 430 the process is clearly explained on page-93.

Make sure your doors are closed.

1) Insert key into ignition and turn to position 2.

2) Within 4 seconds push button "0" and release.(this is the odometer reset button)

3) Within 10 seconds turn key to position 0.

4) Press and hold button "0" while turning the key back to position 2. Continue to hold button "0".

5) After approx 10 seconds a signal sounds and the display will show 10000 miles or 15000 km. Release the button"

-- Anonymous, November 20, 2002