If the music is too long: You can cut the music using the razor blade at the end of the video or you can use the mouse to grab the tick at the bottom-right end of the music track and drag it back to end at the end of the movie.

If the music is too short: Use the Duplicate Object button to make a duplicate of the music track, then put that track under the current one. Push the track to the end of the video so the new track ends with the video.
Now find a point between the two tracks where they can mix well without sounding awkward and cut or blend them there so the top music track ends and the bottom track picks up at that point. This will make the track longer and trail out at the end of the video.
Hope that helps.
J