if you just wana change and save the screen brightness only, you can use xbacklight

     sudo apt-get install xbacklight

after installing, type command to set the screen brightness easily

     xbacklight -set `num`

the num is percentage of your screen brightness.

An easier way to set brightness and contrast

     sudo setpci -s `00:02.0` F4.B=`XX` 

to set brightness, 00:02.0 is your VGA device code.XX is hexadecimal form 00 to FF

use lspci command to find out your VGA device code.

     xgamma -gamma `X`

to set contrast,X from 0 to 1