If you're trying to open up the mouse yourself, note there are (5?) screws at the bottom.
One is immediately visible. 3 are under the rubber feet, and one is hidden inside the battery compartment, covered by the sticker so you need to feel around for the depression to tell where it is. You'll have to poke a hole through the sticker to get to it.
Remove the ball before trying to lift the top cover out.
But I have to agree. This mouse has terrible switches. Everyone else I know who uses it has had the same trouble, with the buttons becoming less responsive/double clicking/etc.
Mine is pretty banged up from having to repeatedly open it up, opening up the microswitches with a sewing needle and spraying contact cleaner (the kind car mechanics use) on the inside. It works but the effect usually only lasts for about 1 or 2 months before I have to do it again.
I'm from a different country so buying stuff from US ebay isn't an option for me. :( I wish I could just replace the switches with nice, durable ones.