I have two Logitech cordless mice - one with a ball and one optical ( which obviously doesn't have a ball ). Assuming yours to similar to the first - the ball is simply extracted by sliding the battery compartment cover forward - the ball should then just fall out and you can get access to the three rollers which are usually coated with gunge. This can carefully be removed with a sharp object like a fingernail ( or you can buy special cleaning kits ). If you have a Optical mouse ( or cleaning the ball rollers doesn't work ) then make sure the 'runners ??' on the bottom of the mouse are also clean.