How do I update my Mac desktop

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.

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Your iMac must be a late 2015 or later model Upgrade to macOS Monterey - Official Apple Support

If you follow the directions in the link I posted you should be good to go. Don't skip the backup before you proceed.