Click on the Apple icon in the top-left of the screen and select “System Preferences”.
Click on the “Software Update” icon
If an update is already available on this window that appears, you should click “Restart now” to begin the update process.
Before doing this, it’s recommended to click the checkbox to “Automatically keep my Mac up to date” so that updates always happen in the future.
Moving forward, you should keep a lookout for any notifications of an update that is available.
The update may take while to complete so it should only do this when you have some free time.