Make sure the ignition is turned off.
Insert the key into the ignition and turn it to the first position (do not start the engine).
Press and hold the button on the right stalk of the steering column (next to the light switch) for about 10 seconds.
While holding the button, turn the key to the ON position and then quickly back to the first position.
Release the button when you see the "Please Wait" message in the instrument cluster.
After a few moments, the service indicator message will disappear, indicating that it has been successfully reset.
Note: Be sure to reset the service indicator before the specified reset interval is exceeded.
Here's a brief video about resetting a Mercedes service indicator: