Software Upgrade on iMac 2012

iMac 27" late 2012 running 10.13.6 can run Mojave macOS 10.14, and tops out at Catalina macOS 10.15.

That iMac cannot run the current release.

How to get old versions of macOS - Apple Support

Mojave macOS 10.14 is the last version that runs 32-bit apps. Catalina requires all apps be 64-bit. That means upgrading from Office 2011 to Office 2016, Office 2019, Office 365, or the upcoming Office 2021, for instance. Upgrading or replacing or retiring all other 32-bit apps, as well. You can see your 32-bit apps using the Go64 tool, among other approaches.

If you're on a deadline right now, go with Mojave.

Have a backup or two created prior to the upgrade. The backup is the only reasonable path to recovery, should there be a problem with the upgrade.

You can upgrade to 10.14.x. We have no idea which version of Mac OS is currently installed on your system but if it's 10.13.6 upgrading is pretty easy. Just follow the steps outlined in: How to get old versions of macOS

No worries, and MrHoffman is 100% correct, you can go to 10.15.7 which is the most current version of Catalina.

One thing you may want to start considering though, your iMac is not 10 years old and it may be wise to start thinking about it's replacement as a 2012 cannot go beyond 10.15.7.