How do I update my Mac desktop

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

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