Upgrade from el capitan
Is it possible to install a newer os on my mid 2011 Mac from el capitan 10.11.6?
iMac 27", OS X 10.11
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The model and year of your Mac will determine which macOS you can upgrade to,
read the following link.
How to upgrade to macOS High Sierra - Apple Support
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
Check that your printers/ scanners etc have updated drivers and software available for the new OS.