To change LCD brightness from command line:
Open your terminal and type this
xrandr -q | grep " connected"
it will gives you the output as eDP-1 connected 1680×1050+0+0 (normal left inverted right x axis y axis) 331mm x 207mm
There eDP-1 Stands for your display .
so now you have to do as
xrandr --output eDP-1 --brightness 0.5
there 0.5 stands for brightness and it ranges from 0.0 to 1.0 . 0.0 -> Full black .so you have to choose the required value of brightness .