I have a Mac Pro mid 2012 running high sierra

That would be good advice for most computers, but the Mac Pro 2010-2012 is not supported to run Catalina, and can only run Mojave after a graphics card upgrade:

Install macOS 10.14 Mojave on Mac Pro (Mid 2010) and Mac Pro (Mid 2012) - Apple Support

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On your version of macOS, the way to get the free update is through the App Store. Launch that and do a search for Catalina in the search field in upper left corner. It is a free upgrade.

Here is a support article with more information: https://www.apple.com/in/macos/how-to-upgrade/

Please be sure to make a full backup of your current hard drive. (Either with Time Machine, or a different service of your choosing)

See Step 2 in these instructions for how to backup your Mac: https://support.apple.com/en-us/HT201475