upgrading IMAC OS High Sierra Ver 10.13.6
How to upgrade IMAC OS High Sierra 10.13.6?
iMac 27″, macOS 10.13
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The model year of an iMac is available at Apple menu > About This Mac.
A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to get old versions of macOS - Apple Support
A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to get old versions of macOS - Apple Support
A mid 2014 and late 2014 can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to get old versions of macOS - Apple Support
A late 2015 or later can be upgraded to the current version of Monterey macOS 12. See: How to upgrade to macOS Monterey - Apple Support