When the brightness of the picture is not uniform across the entire screen of an LED or LCD TV, it means that not all of the backlights are working inside the panel. LED and LCD TV panels contain backlights that produce a field of light that shines through the LCD layer and lights up the picture.
The backlights are made up of columns of CCF lamps or white LEDs and are arranged such that they are spaced about an inch or so apart. It is possible to have only one light go out, or a row, or a section. You can read more about these technologies in Is my TV an LCD, LED or Plasma?
Backlights are available as replacement parts for rebuilding the panel (see Replacement Parts for TVs). And although they can be replaced, this author is not inclined to recommend rebuilding the panel. Not only is it a time consuming task, but there is a risk of either damaging the screen or getting dust or debris between the layers of the screen in the process. It is safer and easier to replace the whole panel. Learn how by going to LCD / LED Screen Replacement.