How do you reset the maintenance service indicator of MB E350 2009?

SOURCE: how to reset service indicator on s 550 m.b., 2007

you need a scan tool snap on motis or deler master tech to reset service light

SOURCE: How do you reset c230 2007 mercedes benz resetting

Try this, it is the procedure for resetting the ASSIST computer on many Mercs
1...Turn Key to position No:2 in the ignition / steering lock and immediately press AND Hold the button "000.0" (the trip reset button) TWICE within the space of 1 (one) second.
2…Turn the key to position No: 0 in the ignition / steering lock. Then release the button.
3...Press AND Hold the button "000.0" (the trip reset button) and turn Key to position No:2 in the ignition / steering lock. Keep button pressed
After about 10 seconds the service indicator will reset and an acoustic signal may briefly sound
That should do it....Good Luck…...Let me know if it works?

SOURCE: How do you reset 2006 ml500 mercedes benz

You will need a code reader/scanner called OBD2. They are under $50. A nice one can cost $100-$3,000. Just buy one that does the job...no need for the more expensive ones. You can borrow one from some auto parts stores, like the AutoZone.
You can also pay a shop this much to do it for you, every time.
Just select erase codes, and you're done.