People who have always used the desktop computers all their life find it very awkward to control the screen brightness when they switch to the laptop computers. In the desktop computers, usually there are physical controls to set the screen brightness, screen contrast and the color intensity. But the laptops do not have any such dedicated controls. You are either supposed to use the Fn keys or find the screen brightness controls buried somewhere deep inside the Windows settings.
If you want to be able to change the screen brightness of your laptop screen, then you can make use of the free software called Win10 BrightnessSlider. Upon launching, it sits in the notification area of Windows desktop and when you click on its icon, it displays a slider control to adjust the screen brightness of your screen.
The program supports multiple screen configurations. So if you have attached an external monitor to your laptop, then you can adjust the screen brightness of all the attached monitors using this tool. If you launched this program first and then attached a monitor and it is not detecting the newly attached monitor then you can right-click on the BrightnessSnider system tray icon and select Detect Monitors to detect all the connected monitors. You can also simply relaunch the program and it will detect all the monitors.
From the right-click on the program icon in the notification area and select Run at Startup to set it to be automatically launched with Windows. This will save you from the trouble of launching the program manually.
As you can guess the program is designed to work only on Windows 10 and the developers are always targeting the newer versions of Windows. You cannot use it on earlier versions of Windows.
You can download Win10 BrightnessSlider from https://github.com/blackholeearth/Win10_BrightnessSlider/releases.