We already know that Android 2.2 brings a new feature that allows you to lock your Froyo based phone’s screen with a password, PIN or pattern. If you have updated your Google Nexus to latest android OS, find here the way to make your phone more secure.
Go to Home screen and select the menu key.
Under settings, select Location and security.
Now tap on Set up screen lock.
You will be prompted with four options: None, Pattern, PIN, and Password.
If you select None, your mobile’s screen will remain unsecured. Choose one of other three options to lock the screen.