How can I update an old iMac 21.5?

Back up your data, check your applications for compatibility, and depending on which iMac model you have, click on High Sierra, Mojave, Catalina, Big Sur, or Monterey. If you get a Mac App Store preview page, change the https at the beginning of its address to macappstore.

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

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How old is your iMac? If you don’t know go to Apple icon - About This Mac and it should be displayed there. I’ll also put a little list for oldest compatible iMacs until

Catalina -

El Capitan - 2007 iMac or later

Sierra - 2009 iMac or later

High Sierra - 2009 iMac or later

Mojave - 2012 iMac or later

Catalina - 2013 iMac or later