Apple Pro Monitors do not come with brightness settings, and all brightness adgustments need to be made with software running on the computer.
All computers with the Apple Pro Monitor should have an applet called StudioBrightness++. To adjust brightness with this tool, click the little arrow in the bottom right corner of your desktop as circled in the image below.
Then, when you click on the studio brightness icon also shown in the image below, a small slider will appear allowing you to set the brightness to the desired level.
Sometimes this app causes the screens to flicker, as automatic brightness is enabled by default. To fix this, just right click on the Studio Brightness icon, and click 'Automatic Brightness' to toggle this setting on.
If the setting is on and active there will be a check to the left of the button, and no check if it is off like in the image below.