Upgrading The Operating System
Why Upgrade?
If your computer's working well, you may decide to leave well alone but Apple do release security updates as well as other important changes so it's worth keeping it as up to date as possible.
Using Software Update
Depending on the age of your machine, you either use Software Update accessed through the Apple icon (top left of the screen) or the App Store utility found in the Applications folder.
This will offer to update you to the latest version of your operating system. If you are currently running Catalina - OS 10.15 then it will not update you to Big Sur - OS 10.16 as that's a separate (free!) download but only the increments within Catalina. So if you are running 10.15.2 you can update free of charge to the latest version available. (At the time of writing, Catalina is at 10.15.7).
So, what happens if you use Software Update or App Store to update the operating system. Well, usually, all goes well and it's a painless operation, but if you have or suspect you have something causing problems, there's another way which may help.