This is how to install macOS updates and new macOS versions. For company devices, please check with support before upgrading to a new major version so that installed programs (e.g. Sophos) are compatible.
Preparation
Back up your data, e.g. with Time Machine.
Connect the power adapter and make sure you have a stable internet connection.
Make sure there is enough free storage space.
Steps
Open Apple menu › System Settings › General › Software Update. On older versions: System Preferences › Software Update.
Wait until the check is complete.
Click Update Now or Upgrade Now and enter your Mac password.
The Mac restarts during installation. This can take some time.
Note: If a new version isn't offered, the Mac doesn't support it or it is held back centrally on company devices.