To delete selectively you need to connect the iPod to the same iTunes Library that was used to add the music to it originally. Before that though, dependant upon the criteria you used to add the music originally (Sync All, or Sync by Playlist), you deselect the music in the Library, that you wish to remove.
If you no longer have access to that library, or you simply want to delete all the music, connect the iPod to your current iTunes library and when iTunes asks you if you wish to delete it all, you answer "yes". If you have a newer Mac, which doesn't use iTunes, then you use Finder, although I'm not sure what the precise procedure is.
Obviously, I've just given a brief outline of how to remove music. For more information, let us know whether you still have access to the same iTunes Library, and how you added the music to these iPods originally (Sync All / Sync by Playlist), and whether you have iTunes or a Mac with Finder.