This used to be in the owners manual but was removed at some point (2000?).
This is for early W210's -
The counter can also be set by any individual. To do so:
1. Turn electronic key in steering lock to position 2.
2. Within 4 seconds press button 0 twice.
3. The present status for days or distance is displayed.Within 10 seconds turn electronic key in steering lock to position 0.
4. Press and hold button 0, while turning electronic key in steering lock to position 2 again. The present status for days or distance is displayed once more. Continue to hold button 0. After approx. 10 seconds a signal sounds, and the display shows 10000 miles (Canada: 15 000 km).
5. Release button 0.
Also I think some minor changes using the steering wheel buttons. On early models button 0 is the one you press and hold to reset the trip odometer to zero.
For some strange reason it seems if you leave the door ajar it doesn't work ??
Sometimes takes two or three tries.