If you mean the music itself, you can just select the songs you want to move in Music (app) drag and drop onto the USB stick (or inside of its open window). That's equivalent of copy from here (Music app) to there (USB stick).
You can also select the songs you want, right click, "Copy", then click inside USB stick window, right click, "paste".
You might want to verify that the car can play AAC files (probably, but not certainly). If not, it probably plays .mp3 and you will need to convert the music files to mp3.