i have a Imac 27 mid 2010 that is running Yosemite

follow this How to get old versions of macOS and scroll to lower region of the web page

Use Safari to download macOS on your Mac

Safari uses these links to find the old installers

in the App Store. After you download the installer from the App Store,

the installer opens automatically.

• macOS Big Sur can upgrade Catalina, Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Catalina can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks
• macOS Mojave can upgrade High Sierra, Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion
• macOS High Sierra can upgrade Sierra, El Capitan, Yosemite, Mavericks, Mountain Lion

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

macOS High Sierra 10.13 hardware requirements

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Back up your data, check your applications for compatibility, and click on High Sierra.

To check the programs, click here and look them up.

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