If you’re running macOS 10.12 Sierra or earlier, and do not wish to upgrade to macOS 10.13 High Sierra, be careful as Apple is now downloading High Sierra update automatically to your Mac in the background, making it all too easy to upgrade inadvertently.
This is a change of behaviour from Apple. In the past, only minor changes or security updates are forced into users like that. Apple explained this change in its support article:
If you’re using OS X El Capitan v10.11.5 or later, High Sierra conveniently downloads in the background, making it even easier to upgrade your Mac. When the download has completed, you receive a notification indicating that High Sierra is ready to be installed. Click Install in the notification to get started.
If you want to install High Sierra later, just dismiss the notification. Install it at any time by opening the file named Install macOS High Sierra from your Applications folder, Launchpad, or Spotlight. Or delete the installer by dragging it to the Trash. You can always get it again from the App Store.
Please note that this ‘convenience’ imposed by Apple is an automatic download of a 5.21 GB Installer in the background. Apple does not bother if you have limited disk space or bandwidth for the download. This could cause users some extra telco fees without them agreeing.
Please note that even if you’ve configured your Mac to prevent downloading updates in the background, the High Sierra Installer will still be forced into you.
If you’re not ready to upgrade to High Sierra, you may want to search under the Applications folder and remove the large “Install macOS High Sierra” file.
macOS High Sierra, together with iOS 11, are the most unpolished releases from Apple in recent years. It is advisable to upgrade to them when their .2 versions are released. Wait for macOS 10.13.2 High Sierra and iOS 11.2.