can i update from El Capitan OS on iMac v10.11.6

Depends on the age of the Mac:

iMac Early 2008 8,1; 9,1 - can only upgrade to 10.11.6

iMac Late 2009 10,1 and 11,1 - can upgrade to 10.13.6

iMac Late 2012 12,1 can upgrade to 10.15.7

iMac 2014 14,4 can upgrade to 11.5.2 Big Sur.

the x,y following the date is the Machine ID found in your System Report Hardware section in Apple menu -> About This Mac. Sometimes called the System Profiler.

Back up your data, check your applications for compatibility, and depending on which iMac model you have, click on High Sierra, Mojave, Catalina, or Big Sur. It needs to be at least a late 2009 one to be capable of a higher OS.

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