Sierra Download

Back up your data, check your applications for compatibility, and depending on which iMac model you have, click on El Capitan or Sierra, High Sierra, Mojave, or Catalina; if you bought it new then, it can run any of those. If you encounter a specific issue, describe it in this thread.

Catalina and newer won't run 32-bit software; download and use Go64 to scan for it. To check other programs, click here and look them up.

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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 Sierra 

macOS High Sierra

macOS Mojave

macOS Catalina is compatible with these computers

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

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal  operation of the OS and may even Corrupt the OS requiring a Reinstallation