How to change lock screen shortcut for Ubuntu 12.10?

Solution 1:

You can do that using the keyboard settings:

Press the Windows key (in Linux world, more correctly called the Super key), then type keyboard.
Click on the Keyboard program that appears first in the list to open it.
From the Shortcuts tab, select System in the left pane.
In the right pane you can change the Lock screen option, just click on it and press any combination of keys that you want.