So you wanna change it in the middle of battle? take a look at yanfly's field states (yanfly.wordpress.com) coz it changes the battleback mid-game depending on field state
but if you simply want to have different battlebacks per battle, you can set a battleback per map by default using the map properties... if the battle is evented, you can also use event commands to change it before the battle... (these two are quite the basics of battles, so if you don't know it, I suggest you get more familiar with the maker for now)
It's called Battleback by the way, not BattleHUD...