1. xrandr -q | grep " connected" 

    and press Enter. Find the external monitor name displayed in this command's output. In my case, it is HDMI-1

  2. To change the brightness enter this command 

    xrandr --output HDMI-1 --brightness 0.5 

    and press Enter. ( change the monitor name HDMI-1 with the one you have ). 
OTHER METHOD

sudo add-apt-repository ppa:apandada1/brightness-controller

sudo apt-get update

sudo apt-get install brightness-controller