Changing Your Screen Lock
From the Home screen, tap the Apps Key
> Settings 
> Lock screen.Tap Select screen lock.
Confirm your unlock information (if necessary).
Tap the desired screen lock option.
You’re prompted to enter your screen lock.
Enter a Backup PIN and tap Continue. Then confirm your Backup PIN and tap OK.
You have 5 opportunities to enter your unlock pattern, PIN, or password. If you used all of the opportunities, you can try again after 30 seconds.
If you have set up a Microsoft Exchange account, you might only be able to use a password.
If you can't remember your unlock sequence, tap Forgot pattern? in the bottom right-hand corner of the screen, then you can unlock the phone using your Google Account (if you created it on the phone) or your Backup PIN that you created when you set up the screen lock sequence.
Enter your Google Account username and password, then tap Sign in.
Enter your Backup PIN, tap Done at the bottom right corner of the keypad, then tap OK.