how to upgrade my imac to Numbers 12.0

To upgrade Numbers just go to the Mac App Store, search for Numbers, and install from there.

According to the Mac App Store to run Numbers 12.0 you need to have updated your OS to macOS 11.0 or later.

If your hardware supports it you can update your macOS to 11.0 or later (I'm at 12.4) via System Preferences > Software Update.

SG

Numbers is a part of macOS, therefore you would have to update first your iMac to the highest possible macOS (12.5) version.

Unfortunately you can not update Numbers Independent from macOS.

Numbers is not part of MacOS but you will have to upgrade from Mac OS 10.2 to install the latest version of Numbers. I do not know what the minimum OS version required is. Maybe 11.