how can i upgrade if i have a imac 27" mid 2011 with OS X Yosemite

A 2011 iMac can only upgrade as far as macOS High Sierra 10.13.6.

Here's how.

Open Safari and click on this link, 

How to get old versions of macOS – Apple Support

Go to the Download OS section and click on macOS High Sierra.

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

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'.

Back up your data, check your applications for compatibility, and click on El Capitan or High Sierra. That model isn't capable of Mojave or above.

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