Terminal: xrandr -q | grep " connected"
Output: eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
So my connected screen name is eDP-1
xrandr --output eDP-1 --brightness 0.5
0.5 = 50% of current brightness. Back to normal use:
xrandr --output eDP-1 --brightness 1
brightness 1 = 100%
Source and thanks to https://askubuntu.com/questions/149054/how-to-change-lcd-brightness-fro…