Well the way I do it is...
1. Go to the event command menu.
2. Chose change BMG.
3. Make sure your battle music/BMG is selected as the one to change.
4. In the scroll box next to it select a new battle BMG.
Usually after the Start Combat command ends I go back and create an event that changes the battle music back to it's normal theme. Make sure you've imported what you want to use as Boss music first though.
I hope this helps