Besides the regular route of using the Mac App Store, it’s possible to use the Terminal to update your Mac. The big difference between using the Mac App Store and the Terminal is that when you use the Terminal method, you can keep using your Mac as the update downloads and the initial installation will take place in the background (via MacRumors).
Update Your Mac with Terminal
You can find the Terminal app by opening Finder and going to Applications > Utilities. Or, you can use Spotlight. Once Terminal is open, type the following command:
Your Mac will search Apple’s servers for any macOS updates currently available. If there aren’t any updates available, it will give you a “No software available” message. If there are updates, it will display them in a list, and you can use the arrow keys to choose it.
To download and install an update, use the following command but replace NAME with the name of the update:
softwareupdate -i NAME
softwareupdate --install NAME
You’ll have to type out the entire thing in single quotes. Example:
softwareupdate --install 'macOS High Sierra 10.13.3 Supplemental Update-'