Why I cannot find update in app store for my macos high Sierra to catalina

Back up your data, check the application compatibility table, click here, and if your Mac is a 2012 or newer model(2013 for desktop Mac Pros), then on the download link for it.

Catalina and above won't run 32-bit software; use Go64 as well.

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Which upgrades—Apple calls bigger and potentially app-breaking changes upgrades, and smaller changes are called updates—are available depends on the Mac model and Mac model year.

Here is: How to download and install macOS - Apple Support

If you want us to check which upgrades are available, post the Mac model and Mac model year from the Apple > About This Mac… menu item.

If it's a late 2013 model, it can run Big Sur; otherwise, proceed as described above.

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