upgrade High Sierra 10.13.6 to Catalina
Is it possible to upgrade from High Sierra 10.13.6 to Catalina using my same Mac?
Do I need to buy a newer Mac?
iMac 21.5″, macOS 10.13
Back up your data, check your applications for compatibility, click here, and if your iMac is a 2012 or newer model, then on the download link for it.
Catalina and higher won't run 32-bit software; download and use Go64 as well as using the first link above.
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