How do i download mac os catalina

Back up your data, check your applications for compatibility, click here, and if your iMac model is listed in the Related Article above, then on the download link for Catalina.

Catalina and newer won't run 32-bit software; download and use Go64 as well as using the first link above.

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See: How to get old versions of macOS - Apple Support

Note that your iMac must be a late-2012 or later model to install Catalina.