If you mean the outside, clean it just like a lens. I use a little cleaning fluid on a Q-Tip to get inside the eyepiece.
If you mean inside, this requires removing the top cover of the camera. You can use lens tissue and a Q-Tip with a little (very little!) cleaning fluid for this. Be very careful of the mirror as it is a front surface mirror and very easily scratched. Less is more!
If you go lightly, you shouldn't mess up the alignment. 😎
P.S. By cleaning fluid, I mean lens cleaning fluid. I like Kodak alcohol free. I find it cleans well and has less problem with streaking.
