The latest available version of software for any iPod is the version shown when you select the iPod in iTunes and go to its Summary screen. It provides the currently installed software version, and if it says "Your iPod software is up to date," that version is the latest version available for your iPod model. If there is a more recent version available, you are be notified there, or you can click the Check for Update button. Any available updates can be installed from that screen.
The software version numbers do not correspond between iPod models, so there may be an iPod nano type with software version number higher than your iPod nano's software, but it is unrelated. The software for a later (or earlier) nano model does not work on your nano model.