OS X Yosemite won't upgrade
I presently have a "iMac 27", with a OS X Yosemite version 10.10.5, that won't upgrade. It hasn't been updated since 2013, when the computer was purchased. Is there any suggestions?
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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: How to get old versions of macOS - Apple Support
A mid-2014 or later can be upgraded to the current version of Big Sur. See: How to upgrade to macOS Big Sur - Apple Support