I've mentioned this here, before:
Your BEST BET is to take the unit, unopened, to a Batteries Plus. I learned their price to change the battery, LABOR INCLUDED, was less than Radio Shaft's price for JUST THE BATTERY. You get the added benefits of being able to do something else while they are working on it (lunch is long enough), and no worries about touching something or doing it wrong and trashing your Ensoniq. I have a ESQ-1, and did it this way. I was quite happy afterwards.
Threre are how-to's on the net about DIY, if just MUST go that way. Google Ensoniq ESQ-1.