How to upgrade from Mountain Lion to El Capitan on Late 2012 iMac?
What is the best way to upgrade from mountain lion to El capitan ?
I am running on iMac (Late 2012)
Thanks
Andy
[Re-Titled by Moderator]
iMac 27″, macOS 15.5
Download the macOS El Capitan installer and create a bootable USB macOS installer on a flash drive.
Create a bootable installer for macOS - Apple Support
If the iMac is bootable now, then use that to create the bootable USB installer. You cannot use a PC or a Mac that can't run El Capitan, i.e. a Mac newer than a 2015 model.
The 2012 iMac can run up to macOS 10.15.7 Catalina. You might consider upgrading the computer to Mojave if you have old 32-bit software you absolutely are married to. Otherwise, to get the longest productive use from the obsolete hardware, you'd want to upgrade to macOS Catalina. The easiest upgrade path to both of those is also to create a bootable USB installer. The guidance for that is also in the doc that I linked above.
Only apps published in 64-bit code are compatible with Catalina and newer macOS versions. If you're interested to know which of your current applications are coded in 32-bits and incompatible with Catalina and later macOS versions, use this free utility> Go64 app from St. Claire Software to check.