Its all in the owners manual, but if you haven't got round to reading it heres a useful video guide showing the 2 ways of resetting a Stihl M-Tronic Chainsaw.
This demonstration was carried out by a Stihl technician during a recent Arbtalk field day at Stihl HQ. Amongst the many topics discussed on the day, one of the most interesting was on the M-Tronic system. Offering a few Arbtalk members an insight into the simplicity of the system, and how to easily diagnose any issues. Stihl have produced a very handy and easy to read article which goes into more detail on the M-Tronic which is available at the following link