How can i change the screen brightness of my laptop in ubuntu?

sudo gedit /etc/default/grub

And then type your root password and insert these lines:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
GRUB_CMDLINE_LINUX="acpi_osi=Linux"

Save and exit.

sudo update-grub

Restart your laptop and then it should work by the "Fn" + left and right arrows.