I assume you have older PS2 or USB Intellimouse.
To take mouse apart remove screws located on the bottom under pads.
(Lift pads gently, later you will glue them back.)
If you have even older mouse then screws are not under pads, you can see them on the bottom.
Take it apart and see if cleaning (washing) solves the problem.
BTW this OEM mouse is $12 online. Check at pricewatch.