I did this a couple of weeks ago, works as good as new now.
http://www.wonderhowto.com/how-to/video/how-to-clean-an-apple-mighty-mouse-scroll-ball-217746/
I assume you mean scroll ball?
Get a proper mouse.
This.
As much as I LOVE my iMac, the MightyMouse is pretty rubbish.
You'll change your mind. It's a terrible piece of mechanical design.