HIGH SIERRA DOWNLOAD

Back up your data, check your applications for compatibility, and if your iMac model is capable of it, click on High Sierra. 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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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

The Only Full Version of High Sierra offered by Apple is High Sierra 10.13.6 and then thee are additional Security Updates

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

You can get a download if you search how to get old versions of macOS. A high Sierra download is available there.