Please I am new to ubuntu 14.04 and i can't adjust my screen brightness. I'm using HP Pro book-450-G2. but i dual boot the ubuntu with windows 10 and my brightness can be adjusted in windows 10.
You can install xbacklight with
sudo apt-get install xbacklight, and then try
xbacklight -set 10.
If that does not work, then open up the folder
/sys/class/backlight/ and see if there are any entries. If not, then you probably don't have your video drivers installed.
Edit: OK, the following seems to have worked for OP: Brightness setting not working on Xubuntu 15.04 ( vivid vervet)