Most likely by replacing the backlight  bulb (or the display panel).  Or is this already a replacement screen (if it is and you're seeing that error, it's likely not compatible with the system)?

Thank you for your reply.  No, this is not a replacement panel.  A back-light bulb, where do I find that in my laptop ?

Inside the display panel.

http://www.laptoprepair101.com/laptop/2007/12/09/replace-laptop-backlight-ccfl-lamp/

Thanks again.  Sorry for the delay.  Prior to me taking the risk of trying to repair anything on the LCD screen, I would like to consider : If the backlight was damaged, wouldn't I notice anything unusual during the regular screen operation ?

Presumably, your reason for posting was that you noticed something abnormal.  Did you not?

What model system do you have - and what type of screen (standard or LED-backlit)?