Generally, if a system upgrade is available for your Mac, it will be in the Apple App Store on your Mac (and its free). The App Store may insist that you upgrade to the most recent version your system can handle, but there are ways around that if you dig on the right tech support web sites. (For example, if you are running 10.11 but your computer can handle 10.14, you may only be offered 10.14 in the App Store, but there are other ways if you only want to upgrade to 10.12 or 10.13.)
