My IMac is 2012. Can I upgrade to OS Big Sur?

Okay I may be posting this a bit late but thought I would

add to the conversation by pointing out a few things you may want to do

before upgrading to macOS Catalina 10.15.7.

Although an iMac from 2012 can upgrade to the newer OS's like macOS Mojave 10.14 and macOS Catalina 10.15, 

I would recommend you upgrade to macOS High Sierra 10.13 first, it has been reported upgrading directly

to Mojave or Catalina directly from older OS's like Mac OS X Yosemite 10.10.5 can sometimes result in the upgrade

stalling usually due to the conversion from the old HFS+ file system to the new APFS file system. High Sierra

should provide the firmware necessary to make the upgrade smoother.

For High Sierra, Mojave or Catalina,

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 free 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.

No. A late-2012 through late-2013 iMac can be upgraded to Catalina 10.15.7. See: How to get old versions of macOS - Apple Support