Remove the battery if it is still installed and leave the battery cover open.

Holding the mouse in your right hand with your thumb at the narrow edge and your index finger near the spot indicated by the blue arrow.

Insert one of the long edges of the guitar pick in the gap indicated by the red arrow and use it to push the top toward the battery cover, at the same time slip the finger nail of your index finder into the gap along the thick edge.

With a little fiddling the wide edge should lift a bit near the blue arrow and the top should pop out of a pin just above the red arrow.

You should now be able to lift the wide edge of the top a little away from the body of the mouse and downwards a bit toward the thin edge releasing the top.

There are two cables attached to the back of the top so just get it loose for now.