How to Change an Apple Mouse Battery: Changing the batteries in your Apple mouse is a quick process that you can do in a few minutes.
Checkout this video:
Open the case of the mouse
To change the battery in your Apple Mouse, begin by flipping the mouse over and locating the small, circular access door on the underside. Use a coin or other small, flat object to twist the door open. If the door is tight, try gently prying it open with a butter knife or other slim tool. Once the door is opened, you should see the battery inside
Take out the old battery
To remove the old battery, first flip over the mouse. There is a small door on the bottom of the mouse that you will need to slide open. Once the door is open, the battery will be visible. Use your fingernail or a small flathead screwdriver to carefully remove the old battery.
Place the new battery in the mouse
1. Hold the mouse upside down in your non-dominant hand.
2. Squeeze the top and bottom of the mouse together with your thumb and forefinger.
3. The back of the mouse should now pop open, revealing the battery compartment.
4. Remove the old battery from the compartment by gently pushing it up with your thumb.
5. Insert the new battery into the compartment, positive side up.
6. Push the back of the mouse closed until it snaps into place.
Close the case of the mouse
Remove the two screws on the underside of the mouse, using a Phillips head screwdriver. There is one screw near the front of the mouse, and one near the back. Gently pull the top half of the case away from the bottom half. The batteries are located in the bottom half of the case.