The screws are under the shiny crescent pads, you can just peal the pads off to get to the screws.
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Most mice have a small phillips screw underneath. Remove this and the mouse should hinge open at the front, at which point you should be able to see the PCB with all the components on it. It's possible that if it's a wireless mouse they've hidden the screw underneath the batteries, this is the case for my logitech wireless mouse.