Yahoo Mail app on iMac?

What is the model year of your iMac? The model year of an iMac is available at Apple menu > About This Mac.

A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. 

A late 2015 or mid 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. 

A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. 

For all of the above, see instructions in: How to download and install macOS - Apple Support

A 2019 or later iMac can be upgraded to the current version of Sonoma macOS 14. See: How to upgrade to macOS Sonoma - Apple Support

You can download it from the App Store on your Mac and try it. Note that you can use both it and the Apple Mail app so if you don't like it you can uninstall it. Not the following from the Compatibility section of the App Store: "Requires macOS 12.0 or later and a Mac with Apple M1 chip or later."