More importantly, do you have a laptop or a tablet computer? If not, the Brightness setting is
NOT available in
W10 Action Center, or if it's there, it's greyed out and you can't change anything. If it's a desktop PC, then Mike's suggestion of finding your video card control panel or downloading it from your video card maker's site (ATI, Radeon, Nvidia, etc.) to control this feature is necessary.
If you are on a laptop/tablet, right-click or swipe from the lower right on the Notifications icon in the system tray (to the right of the time & date), hover your mouse on the Notifications icon, and click on the
OPEN ACTION CENTER item on the task menu that shows up. In the Action Center mini-tiles that show up on the lower-right portion of your desktop, you'll see one that has the
Brightness Icon (I can't show that one unless I take a screenshot), but it will say either
100% on the tile next to a tiny light-bulb icon if your screen display is currently set to 100%; alternatively if you've changed it using function or windows-key-logo combo keys with the function keys (laptop only) it might display at
0%,
25%, 50%, or 75%. In these cases, clicking successively on the Brightness mini-tile with your mouse incrementally changes the screen brightness on your display instantly!!

Cycling through, starting at 100%, it would be 0% (dimmest display), 25%, 50%, 75%, and finally clicking again will bring you full circle back to 100% brightness!

This works perfectly on my 8 year old Sony Vaio laptop.
Desktop PCs are much more difficult to change this setting on, as we said you need to find or download your video card maker's control panel utility in order to change this; W10 won't allow you to do it. If you have a laptop/tablet, we just solved your problem!
Best,
<<<<BIGBEARJEDI>>>>