Hi mydavid0615,
Without knowing the manufacturer, make and model of the mouse, it's hard to tell. They're all different.
But that said, most I've seen have a button or a push bar underneath which when you move it as far as it will go will release the casing cover that holds the batteries (which I've found is normally on top at the back end of the mouse, but you can't and shouldn't try to get it to open until you release it as noted).
If that doesn't help, then please provide us with the above information about the mouse and we'll try to assist you from information on its website.
Good luck!