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You should back your data up before updating software.
Open System
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Settings and navigate to General → Software Update.
Click Update Now or Upgrade Now.
“Update Now” installs the latest updates for the currently installed version. This is a patch update.
“Upgrade Now” installs a major version with a new name.
Software Update will download the new version of macOS, which can sometimes be a large download that takes significant time. It will restart several times and the computer will not be usable while the update is in progress.
Once you see your Mac’s login screen, the software update has completed.