Select menu : About This Mac. On that panel, you will see the specific version of OS X that you are running, and there will also be a Mac model description line including content in parenthesis (e.g. Late-2013, etc.). What are these displayed items?
The Mac model information will help determine your hardware compatibility to upgrade to newer operating systems:
How to upgrade to macOS El Capitan
How to upgrade to macOS High Sierra
How to upgrade to macOS Mojave
Before any upgrade to a newer version of macOS, you should verify with the vendor that your third-party software packages are still compatible with the operating system that you plan to upgrade too, or if they require a minimum version of the operating system for installation. Likewise, confirm that your printer/scanner vendor has drivers and support software that are compatible with the intended upgrade operating system. If not, there is no assurance that the devices will work with outdated drivers.
Also before you upgrade, perform a full Time Machine backup of you Mac, or manually run it once just before the upgrade is commenced. Eject and disconnect the Time Machine drive before the update as you do not want Time Machine mingling the new operating system onto the old backup. You will want a new Time Machine drive (2.5 - 3x startup drive size) exclusively for the first new operating system backup.