How to clean my Nikon D3100 focusing screen?

There's some dust (I'm sure of it) on it. How do I take it out and clean it? I have a rocket blower.

Fire the shutter at medium speed a couple of times with the lens off and the mount pointing down - then use the blower to gently take off what's left.

I don't recommend you do this yourself. Take the camera to a pro to have it cleaned. They have all the correct equipment, knowledge and experience that you don't have. You could mess up your camera permanently.