How can the DCS816 be reset to default data, if the tech passcode is not known?
If you want to default the whole system then you simply turn the battery backup switch inside the system to off, then power off system. When you turn system back on it will be in default condition. You will then need to reprogram system but the tech password will be at the default.
If you want to default or change the tech password if unknown you can contact Samsung Tech Support in your country.