Find your laptop graphics card in /sys/devices
On my system, the internal, laptop screen is called eDP-1, so I
search for card1-eDP-1:
$ find /sys/devices -name card1-eDP-1
/sys/devices/pci0000:00/0000:00:02.0/drm/card1/card1-eDP-1
If you get no hits, search for card1 instead and see what cards are present:
$ find /sys/devices -name card1
/sys/devices/pci0000:00/0000:00:02.0/drm/card1
When you've found it, cd into the dir and follow the steps below:
$ cd /sys/devices/pci0000:00/0000:00:02.0/drm/card1/card1-eDP-1
Find the max brightness
$ cat ./intel_backlight/max_brightness
1060
Find the current brightness
$ cat ./intel_backlight/brightness
1060
Adust the brightness
Here, I decrease the brightness, which was at 1060:
$ echo 900 | sudo tee ./intel_backlight/brightness
900
Set max brightness
$ cat ./intel_backlight/max_brightness | sudo tee ./intel_backlight/brightness
960
