Hard Reset (Factory Reset)
If you forget your lock sequence and backup PIN, you will have to perform a hard reset to gain access to your phone.
Performing a hard reset deletes all of your user data, which includes pictures, videos, and music saved to your internal memory. Please make sure to back up your information prior to performing a hard reset.
Turn the phone off.
Press and hold the following keys at the same time: Volume Down Key
+ Power/Lock
Key 
.Release the keys when the FACTORY HARD RESET screen is displayed.
Press the Power/Lock Key
to continue or one of
the Volume Keys 
to cancel.Press the Power/Lock Key
once more to confirm
or one of the Volume Keys to cancel.
You can access the Factory data reset menu via the UI through
the Settings menu. From the Home screen, tap the Menu Key 
> Settings > Backup & reset > Factory data reset.