How do you reset brake wear warning message on 2009 c300 Benz?

Replace the brake sensor then the message will go away. When you saw the message the sensor is already bad so it need to be replaced along with the brake pads. You don't need to replace front and rear sensors at the same time. Any of them has the pads wearing too much (usually in the rear because Mercedes cars drive by the rear), that where the sensor need to be replaced. There is no electronic reset for brake pad wear message.