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System Preferences on your Mac
System Preferences is where you personalize your iMac settings. For example, use Energy Saver preferences to change sleep settings. Or use Desktop & Screen Saver preferences to add a desktop background or choose a screen saver.
Customize your iMac. Click the System Preferences icon 
in the Dock, or choose Apple menu 
> System Preferences. Then click the type of preference you want to set. To learn more, see Customize your Mac with System Preferences in the macOS User Guide.
Update macOS. In System Preferences, click Software Update to see if your Mac is running the latest version of macOS software. You can specify options for automatic software updates.