I am also having this same problem. My brightness was set to max after updating and it hurts my eyes. I did this.....Control Panel > Device Manager > Display Adapters > Intel HD Graphics 4000 > Driver > Update Driver > Browse my computer > Let me pick. Choose Microsoft Basic Display Adapter. After doing this my problem was fixed and I was able to adjust the brightness of my screen again. BUT if you play games on your laptop, doing this will cause display problems within your game.      
Hello. I Have a Dell XPS 15 L521X.   Recently updated to Windows 10 and I can no longer adjust my brightness up or down.   Right now the display is pretty dim.  I would guess about 30% - 40% brightness.  In Widows 10 I found about 5 different places to adjust your brightness and I made sure all of them were set to 100%.  I have my power plan set to high performance and I went into the advanced settings and turned off adaptive brightness and made sure both display brightness & dimmed display brightness are both set to 100% on battery and plugged in.  Rebooted and still dim.   My question is, on Dell laptops, should Windows have full access to control brightness?  Or does Dell override Windows with their custom software for display settings?   When I bought this laptop in 2012, the software, at the time, was called Dell QuickSet.  Is that the application responsible for controlling screen brightness?  And is it required?    Thank you, Mike
Can't adjust brightness after installing Windows 10