1) Right-click the desktop and select personalize
2) Choose Lock Screen from the side bar.
3) Choose screen saver setting on the bottom.
4) From this screen choose your screen saver of choice then click preview.
5) Click to stop the preview, click apply, and then click the OK button.
Under the Lock Screen side tab, you can customize the lock screen picture. You can also hit the Windows key+L to quickly lock your screen.