Look at the bottom of the switch and you'll see 3 black tabs on the plastic that covers the bottom of it. Those 3 taps can be pressed in so the whole assembly can be removed from the body of the switch. Pay attention as it comes apart so you can put it back together the same way. There is 2 spring loaded brass plates under the main contact area.
Before you try to remove the plastic, heat it up with a hair drier or heat gun so the plastic tabs don't break off. You can also dip the end into boiling water to soften the plastic. Since you're going to clean it up anyway the water won't matter.
To get the switch cover off or on, put the switch in the ON position so that the recesses line up. Otherwise the plastic tabs won't push in. You may need to slightly move the switch to align each tab better. They all don't seam to line up in the same spot exactly.
The metal turn shaft only lines up one way in the switch. Notice that one side of it has a narrower side. The hole in the plastic has the same thing so It lines up.
Clean all the contacts and plates up with a wire brush and or fine sandpaper. Be sure the assembly is clean. I use a coat of dielectric grease on all the contacts before I assemble it. This will add to the life of the unit by keeping it from corroding in the future.