how do i upgrade from a 2012 mac

From the Apple logo on the menu bar, select About This Mac. On that panel, what is the Mac product description line including the content in parenthesis? How much memory is shown, and on the Storage panel, do you have Flash storage, or not?

See which operating system you can upgrade to based on your Mac model, and year qualifier (e.g. mid-2012, etc.). Note that your Mac is too old to upgrade to macOS 11 (Big Sur), and that macOS Catalina does not support any 32-bit applications, or 64-bit applications with 32-bit helper components. That means no non-subscription Adobe software, no Apple iPhoto, no Apple iWorks '09 or earlier, no Microsoft Office for Mac 2008/2011, etc. If this is an issue, then do not upgrade beyond macOS Mojave (10.14.6).

Ordinarily, I would recommend St. Claire Software's free Go64 application which would help you detect software that is incompatible with Catalina, but the oldest operating system support that I could find on their site is newer than Mavericks (10.9.5).

For an eight-year old Mac, I would recommend that you boot into Recovery, and run Disk Utility First Aid on the startup drive, and fully remove per vendor's instructions, any anti-virus, or so-called cleaner applications. They are unnecessary. Be absolutely certain to make a Time Machine backup prior to any upgrade to a newer operating system. Check with printer, scanner, and third-party application vendors if there is support for the newer operating system that you select — before you make the leap.

Also go to the Apple in the upper left corner and then "About this mac" and tell us what it says for memory. If all you have is 4 GB then I would not recommend going higher than El Capitan unless all you do is very, very light use. Unfortunately upgrading RAM memory in a 2012 iMac is a 30 step process with many potential pitfalls. While El Capitan is not close to Apple's "latest and greatest", it is still not that far behind and is at least seeing some software updates supported by third party vendors.