upgrade MacOS high sierra
how to upgrade MacOS high Sierra to current Macos
iMac 27″ 5K, 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.
A late-2012 through late-2013 iMac can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683
A mid-2014 or later can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT201475