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System Settings on your Mac
System Settings is where you personalize your iMac settings. For example, use Displays settings to change the brightness or resolution of your display. Or use Wallpaper settings to add a desktop background.
Customize your iMac. Click the System Settings icon 
in the Dock, or choose Apple menu 
> System Settings, then click the setting in the sidebar you want to adjust. To see additional settings, you may need to scroll down. To learn more, see Customize your Mac with System Settings in the macOS User Guide.
Update macOS. In System Settings, click General, then click Software Update to see if your Mac is running the latest version of macOS software. You can specify options for automatic software updates.