How do you adjust the back light on a jvc tv?

I'm in the picture settings on my jvc LT-49C870[A] TV all the different picture settings are all high lighted apart from the back light adjusting bar, it says in the manual you can only adjust this when the above energy settings are in custom above,gone to the settings above and changed them to every setting but the back light one still won't high light any help please thanks in advance (there's a lot on this tv that won't do what they said it would do like link to bbc Iplayer free view and the list goes on )

Daniel
September 2018