What is the latest update available for 2012 MOS
How can I install Big Sur on my 2012 MacOS
iMac 27″, macOS 10.15
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A late 2012 iMac can be upgraded to Catalina 10.15.7 but no later.
Also see the following from Identify your iMac model - Apple Support
iMac (21.5-inch, Late 2012)
Model Identifier: iMac13,1
Part Number: MD093xx/A, MD094xx/A
Newest compatible operating system: macOS Catalina