First method:

1. Start by turning off the phone.
2. Then press the Power Button once.
3. Afterwards press and hold the Volume Down button.
4. Release held key when you see the exclamation point.
5. Press the following sequence of buttons: Volume Up - > Volume Down-> Power - > Volume Down.
6. Success! The hard rest has just been performed.

Second method:

1. If the display is off, press the Power to turn it back on.
2. In the nest step fo to Windows Phone Settings.
3. Afterwards find and select About.
4. To confirm the whole operation tap Reset your phone.
5. Great job! The factory data reset is done!