I'm assuming you are having difficulty PSYCHICALLY accessing the device and replacing the CMOS battery?
[https://www.youtube.com/watch?v=wMms2Y2A_H8|This video] shows how to remove the top, I think there are screws at the top, back that need to be removed first.
I'll add that a symptom of a dead cmos battery is a constant need to input time and date on start-up, is that correct for your device?