Change Magic Mouse batteries
If your mouse uses replaceable batteries, replace all of them at the same time with the same kind of battery. The Magic Mouse uses two AA batteries — lithium, alkaline or rechargeable. If you’re using rechargeable batteries, recharge them all at the same time.
Dispose of the old batteries according to local laws and regulations.
Remove the battery compartment cover (located on the bottom side of the mouse), then remove the old batteries.
Insert two AA batteries, making sure the positive and negative ends are pointing in the right direction.
Replace the battery compartment cover.
Turn the mouse on when you’re ready to use it.
If you previously connected your mouse with your Mac, you do not have to set up the mouse again after replacing the batteries. The mouse should automatically connect itself with your Mac a few moments after you turn it on.
If your mouse isn’t connected with your computer, see Connect an Apple wireless accessory with Mac.