In order to hard reset a Sony Ericsson W850, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Sony Ericsson W850 device will be brought back to the default configuration of a brand new device.
1. Switch on the phone.
2. Go to Menu > Settings > Backup & Reset.
3. Continue by selecting Factory Data Reset > Reset Phone > Erase Everything > Ok.
4. Now just allow the phone to reset'n'restart. Done!