want to upgrade my OS X El Capitan iMac 21.5 inch mid 2011 to latest OS
suitable OS for mi iMac mid 2011, current OS X El Capitan
iMac 21.5″, OS X 10.11
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A 2011 iMac can only upgrade as far as High Sierra.
Open Safari, Firefox will not work for this,
click on the link below,
How to upgrade to macOS High Sierra - Apple Support
Go to Section 4 and click on Get macOS High Sierra.
This will redirect you to the App Store High Sierra download page,
it should say Get or Download.
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.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS High Sierra.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
The newer OS's require macs to be from 2012 or newer and have 'Metal' capable graphics cards.