Need information on how to download and install Software update for MacBookPro9,2

Your mid-2012 MBP can upgrade to macOS Catalina, but no further. Catalina only supports 64-bit applications and drivers, so if you choose Catalina, you may be faced with some applications that won't work, or loss of printer/scanner driver support.

Check what 32-bit applications you have with the free Go64 app.

You can get macOS Catalina or Mojave (32/64-bit app support) via the How to get old versions of macOS - Apple Support site. Use Safari only to access that site, and you will find links to the Mac App Store for Mojave or Catalina there. These will download and reside in your /Applications folder as normal installers.

Recommend that you make a Time Machine backup, and verify with your third-party application/driver vendors if they provide support for the intended operating system upgrade — before you do so.

Which app specifically?

You are welcome.