Can a Mac operating system be upgraded?

Back up your data, check your applications for compatibility, and depending on which Mac model you have, download El Capitan, High Sierra, Catalina, or Big Sur. If it's incapable of running the one you're trying to upgrade to, you need a newer computer.

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What is your computer's SUB-MODEL? From "About this Mac..."

Sub-model is "Mid 2010" in the example. What does yours show?

That means everything to what OS you can use, as older Macs cannot run the latest OS versions.