Performing a Hard Reset (Factory Reset)
If your phone does not restore to its original condition after an error or you forgot your screen lock, perform a Hard Reset (Factory Reset) to initialize it.
If you perform a Hard Reset, all user apps, user data, and DRM licenses will be deleted. Please remember to backup any important data before performing a Hard Reset.
Turn the phone off.
Press and hold the Power/Lock key
(on the right side of
the phone) and the Down Volume key 
(on the left side of
the phone) at the same time.When the Factory data reset screen appears, release both of the keys.
Use the Volume keys
to highlight Factory data reset, then press the Power/Lock
key to confirm.Use the Volume keys
to highlight Yes, then press the Power/Lock key to confirm
one more time.Your phone will perform a factory reset.