OS system update
Can I update Mac OS High Sierra 10.13.6 to a later version?
iMac 27″, macOS 10.13
Back up your data, check the application compatibility table, click here, and if your iMac is a 2012 or newer model, then on the download link for a newer OS.
Catalina and higher won't run 32-bit software; if you upgrade to one of them, use Go64 as well.
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