Switch you phone
Press and hold the Volume Up + Power button to enter the recovery mode
use the volume buttons to move the highlighter and the power button to confirm selection. On the recovery mode scroll down and select "wipe data/factory reset". and on the next screen scroll down again and select "Yes----delete all user data". Wait until its says complete and then You can reboot your phone.
Make sure your phone is 100% battery full.
Please feel free to comment below' if further assistance is needed.