My system is MACOS High Sierra doesn't have any updates Is my Mac pro too old, bought it in 2013

Back up your data, check your applications for compatibility, click here, and then on the download link for Mojave, Catalina, or Big Sur. They're full-version upgrades and won't appear in the Updates list.

Catalina and newer won't run 32-bit software; download and use Go64 as well as using the first link above.

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they can be downloaded from the Mac App Store.