updates for iMac

The model year of an iMac is available at Apple menu > About This Mac.

A late-2009 through mid-2011 iMac can be upgraded to High Sierra 10.13.6 but no later version.

A late-2012 through late-2013 iMac can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683

A mid-2014 or later can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT201475

Hello! It looks like the latest macOS release your iMac can run is macOS Catalina. This is detailed in the Catalina compatibility information here: macOS Catalina is compatible with these computers - Apple Support.

An iMac from 2013 can upgrade to macOS Mojave 10.14.6 or macOS Catalina 10.15.7

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 the OS you want to install.

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 ******.app.

Move it to the Trash, empty the Trash.

Restart, click on the link again, the App Store should say ‘Get' or 'Download'.

If you are considering upgrading to Catalina be warned it does not support 32bit apps

Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,

some updates may be free, some may be paid. Use this app to discover which apps you have

are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see

if they are offering 64bit updates.