Hard reset
Try this,
1. With the phone powered off, press and hold the Power and Volume Down buttons simultaneously.
2. When the Factory Hard Reset screen appears, release the buttons.
3. Press the Power button, then press Power again to confirm your selection.
Be patient as the phone performs all steps and restarts.
Or if you have access inside of the phone try this,
From Android OS
1. Select Settings > Storage > Factory data reset.
2. Check the Erase internal memory box if you wish to delete personal data like music and photos.
3. Tap Reset phone > Erase everything.
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