Update software on Mac

You should go into the App Store and Upgrade.

This is a great guide that covers the upgrade

https://www.apple.com/macos/how-to-upgrade/

FWIW, newer OS versions are upgrades, not updates.

A late 2012 iMac can run Catalina; however you best check that you have a minimum 8 GB RAM or the experience will not be good, the new OS versions require much more RAM. Also, there are major system changes in Catalina - none of your old 32 bit apps will work, so you need to either upgrade them, buy new ones or different apps if necessary. Check all of that thoroughly before you take the plunge.

Also make sure that you have at least one good complete backup - a bootable clone would be best - just in case the new OS is not working as it should, so you can easily boot from that clone, wipe yout hard drive, and clone back your old system.

What model/year is your device?

And which Mac OS version are you running?

If this is an 21.5-inch iMac, it is not possible to increase the RAM without profession help.