Switch off the phone.

Pres the Power button and when you feel a vibration release it and press and hold the Volume Up button.

From Boot mode menu select [Recovery mode] (use Volume Up for navigation and volume doun for OK).

Now you are in the known Android System recovery menu. Use again the Volume Up for navigation and the Volume Down for OK and select one by one: wipe data/factory reset - Yes -- delete all user data - reboot system now.

Tis will reboot the device and will restore to its default settings.