As you can see in this picture there are six screws, just cut a tiny hole with a knife in those places to reach the screws. To get to the two screws under the rubber part, you can just fold them a little from both sides. Onces you have it opened you can put a little vaseline (petroleum jelly) on the thing that holds the mousewheel, turn the mousewheel a few times so you know it stopped squeeking and then remove the unnecessery vaseline.
Then screw it back together!
Hope this helped!