On my laptop it's a blue function button (Fn) on the bottom left that needs holding down whilst you use a couple of the cursor keys (arrows) to adjust. On some I've seen, the adjustment is using the numbers (the ones above the letters, not the numeric keypad) instead of the cursor keys.
Failing that, you could try looking for a manual via Google.
One thing worth remembering on laptops is the screen brightness is usually controlled by the power settings, ie brighter on mains power, darker on battery.
Worth checking control panel power settings to tune it for your usage.