I need to update my OS from Sierra on an old Mac

Go to the Menu, About This Mac, and click Software Update. It will tell you if it can be updated, and the highest version that it can be updated to.

You posted in the iPhone forum; I will ask the hosts to relocate your post to the Mac forum.

Back up your data, check your applications for compatibility, and click on High Sierra. While a 2010 desktop Mac Pro can run Mojave, they need to have a Metal-capable graphics card and go through High Sierra first. If you get a Mac App Store preview page, change the https at the beginning of its address to macappstore.

To look up the programs, click here.

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macOS Monterey supports most Mac models introduced in 2015 and later, with additional support for Mac Pro and Mac mini.

View compatible models