I don't really know anything about the battery apart from that it is 4.2v, 800mA. I doubt you can change the battery as the case is fully sealed with no screws to get into it but i'm not about to find out, lol.
Just to mention aswell, it has got an on and off switch on the bottom incase you don't use it for a long period of time, so obviously this will save the battery life.
I always replace my mice every year or so anyway as there are new ones coming out all the time.