You can perform a factory reset right from the hardware, the menu settings and from the web-interface.
Hardware factory reset:
By pushing the reset button on the back of the unit, hold for a few seconds and the system will reset.
This will also reset the default password to the device to 0000, when accessing the menu.
Reset button on the Yealink VC110
Reset button on the Yealink VC120
Menu factory reset:
Web-interface factory reset:
Log into the web interface ( default Username: admin, password: 0000 )
Settings > Upgrade > Reset to Factory Settings or Reboot