I have a Dell XPS 13 (2016 version) and the brightness Fn keys suddenly stopped working. I updated my display driver, but the buttons still don't work. When I press one of them, my brightness indicator shows up in the top left corner of my screen, but it doesn't move. The rest of my Fn keys work fine, only the brightness keys don't work. Windows updated recently and I'm on the Slow channel of the Windows Insider Program, so I'm thinking there's a bug in that update that broke my brightness keys. Is there a way I can fix them?
More than likely you need to reinstall the software that controls the brightness keys from Dell support page. Have it auto detect your computer if you want. In the Drivers & software section, you will find driver/software updates. Read their descriptions thoroughly and you should find the one you need to the brightness controls.