can a late 2012 iMac have an OS upgrade

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. Make sure to use Only Safari to commence the download as Other may not work

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is not workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs 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.

Safari on older versions may not work. I have seen the download link on updates completely disappear from Apple's page on older versions of Safari. Keep in mind there are other browsers which may work, and you'll want to test them:

https://discussions.apple.com/docs/DOC-250003586

Back up your data, check your applications for compatibility, and click on El Capitan, High Sierra, Mojave, or Catalina.

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