Apple ships regular point releases (27.1, 27.2 and so on), smaller security responses and, each year, a major new version. Keeping up to date is one of the most effective things you can do for security.
1Automatic updates
Open System Settings > General > Software Update and click the info button next to Automatic Updates. A sensible setup is to check for updates, download them, and install security responses and system files automatically. You can still choose when to install the macOS update itself.
2Update from Terminal
softwareupdate --list
sudo softwareupdate --install --all --restart
softwareupdate --history3Apps
App Store apps update from the App Store. Other apps usually have a Check for Updates option in their app menu. Homebrew users run brew upgrade.
4Before a major upgrade
Back up with Time Machine, and check the backup finished.
Check compatibility of apps and hardware you rely on, especially audio interfaces, VPNs and security software.
Free space: leave at least 25 to 30 GB free.
Wait a little if you depend on the Mac for work. Many people wait for the first point release.
Plug in laptops and leave plenty of time.
5Do the upgrade
Major upgrades appear in Software Update. Install, let the Mac restart (possibly more than once), then sign in and let it finish background tasks. Afterwards, run Software Update again to catch any follow-up fixes.