How can I update the firmware of my late 2012 iMac 27” computer?

If you really mean to ask about upgrading the macOS software, then this:

The macOS version you can upgrade to depends on the year your Mac was released.

The 2012 iMac cannot be upgraded beyond macOS 10.15.7 Catalina.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

➡️ macOS 12 Monterey is compatible with the Macs listed here

Follow up -

Fyi, when required, firmware updates for the hardware are usually rolled into and included in macOS software updates, thus killing two birds with one stone. Occasionally, Apple may issue a stand alone firmware update. You can browse for more info here > Apple - Support - Downloads

Why do you feel your firmware is out of date?