ClickMonitorDDC is a freeware tool which allows adjusting the brightness and contrast of DDC-compatible monitors (most of them, laptops too).
You could do this anyway with the monitor buttons, of course, but it's often easier to use the mouse or keyboard, and you're also able to set brightness or contrast to a fixed value. (If you know 70/100 is correct, just click "70", no need to manually adjust each time.)
The program arrives as a single tiny executable, no installation required. Simply run it and an icon appears in your system tray showing the current brightness value.
Click the icon and sliders appear for brightness, contrast (on desktops only, not laptops), and there's a volume slider as a bonus.
Clicking and dragging a slider adjusts brightness and contrast in real time. Alternatively, clicking one of the numbers next to the slider - 5, 10, 15 etc - sets your display to that value immediately.
If you have multiple monitors then you should be able to select another, and adjust that separately.
A hotkey feature allows all of this to be tweaked from the keyboard. You're able to assign brightness, contrast and volume values, or just change one or two, and it's even possible to tweak these values incrementally (increase brightness by 6, decrease volume by 8 and increase volume by 10%, say).