Cutting down on the number of wires that must be tolerated in a work station adds to maneuverability and ease of location. For all their convenience, though, wireless accessories bring with them a number of issues not present in their wired counterparts, most significantly battery change.
Turn off the mouse.
Remove the top of the top of the battery compartment. The button that releases it is located toward the rear of the mouse.
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Remove the old battery and place the new one in its place. Replace the cover and turn the mouse back on.