Hello Papa,
Good day!
I'm Jericho, a fellow customer and an individual advisor. I am happy to assist you!
With regards to your concern, you may adjust the brightness by directly adjusting them in the monitor buttons. Usually, there is a menu button for monitors wherein you could adjust the size and the brightness or contrast of the screen.
You might not see the Change brightness for the built-in display slider or your main monitor on desktop PCs that have an external monitor.
To change the brightness on an external monitor, use the buttons on it.
https://support.microsoft.com/en-ph/help/402694....
Hope that helps! Please let me know if you have other queries/concerns.