Mac OS for iMac late 2012

I have replaced a failed hard drive on my 21.5 inch A1 iMac (late 2012) and have installed Mountain Lion 10.8.5. Before the drive failed the iMac was running a more recent version of Mac OS. I am unable to update the mac os since hard drive replacement. According to the apple website this machine should be able to update to Catalina 10.17.7. Is there a way to update the OS to Catalina on this machine if Software update does not show it?

Thanks