New To Mac - Help And Support For Apple Computer Users

Upgrading The Operating System

This procedure is a "belt and braces" alternative to Software Update or App Store and describes the procedure for modern versions of macOS.

Take a backup of your important data. (See Hard Disc Backup)
Restart your computer whilst holding down the alt (option) key. This will show you all the discs capable of starting your Mac.
Click on the Recovery disc and then the arrow underneath to start from it.
Select Disc Utility and click Continue.
Select your start up volume (your hard disc - probably called Macintosh HD) and click on the First Aid tab.
The reason you started from the Recovery disc is that Disk Utility cannot repair the disc it is being run from i.e. having started from your normal startup disc, Macintosh HD, it cannot repair itself.
Disk Utility will either report that it has found errors and fixed them all or that no errors were found. If it did find errors, run it again until it reports that no errors were found.
Go to the Apple logo and restart normally from your internal hard disc
Download and run the latest Combo Updater for your version of the OS.

The Combo Updater is available as a download from the Apple web site.