Update operating system on Mac Pro (2021

Back up your data, check the application compatibility table, click here, and then on the download link for a newer OS.

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Apple did not release any Mac Pro minitowers or rack-mount computers in 2021.

So, by any chance, are you referring to a MacBook Pro notebook? All of those with a hardware model year of 2021 had M1, M1 Pro, and M1 Max processors – and can be upgraded all the way to macOS 26 (Tahoe). I do not believe there is an App Store link for Tahoe, so if you wanted to install it, you could use Software Update (as outlined in the article to which Niel linked).