Take a back up of the phone (or copy anything you want to keep to your PC or SD card) then do a factory reset from recovery menu.
This will wipe the phone and restore to factory settings.
With phone off hold together Vol Up + Home + Power keys.
Let go when phone loads bootloader menu.
Navigate to Reset/Clear User Data with Vol down key and press power key.
Press power key when the screen returns to reboot the phone.
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