Changing your Lock Screen PIN or Password
From the Home screen, tap Apps
> Settings 
> Lock screen.Tap Select screen lock.
Enter your current lock sequence.
Tap PIN to change your number lock sequence or tap Password to change your alphanumeric lock sequence.
You’re prompted to enter and reenter your new lock sequence.
From then on, you will need to enter the sequence and tap OK to unlock the screen.
You have 5 opportunities to enter your unlock pattern, PIN or password. If you used all of opportunities, you can try again after 30 seconds.
If you have set up a Microsoft Exchange account, you are only allowed to set up a password.
When you can’t recall your unlock pattern:
If you’ve created a Google Account on the phone, tap Forgot pattern?. Then you are required to sign in with your Google Account to unlock your phone.
If you forgot the unlock pattern, you can unlock with Backup Pin.