A very useful website:
How to reset Mercedes Benz A-Class Service Indicator
Turn the key to position 2 in the ignition lock and immediately press button 000.0 twice within the space of one second.
Turn the key to position 0 in the ignition lock.
Press and hold button 000.0 Turn the key to position 2 in the ignition lock. Keep button 000.0 pressed.
Service message with current remaining distance or remaining time appears.
After about 10 seconds, an acoustic signal will sound and the service display with the new starting distance or starting time appears for about 10 seconds.
Release button 000.0
Thats it youve reset your service indicator.