Acer Liquid MT S120 - Manual (Page 51)

51 Resetting your smartphone
Open Settings from the application menu, then tap Privacy > Factory data reset;
click Reset phone and then Erase everything to format your phone.
If your phone is not responding to screen input
Turn off your phone, then press and hold the power until you feel your
smartphone vibrate and/or you see the Acer logo screen. Press and hold the
Volume up and Volume down buttons. Use the Volume down button to select
Clean boot: Yes. Press the camera button to confirm.