Hi Pretesh, a Nikon AF-S Nikkor is a lens, not a camera. I have no idea exactly what camera you have but on the D40 there is a large button to the left of the lens. You need to push this button in and then grab hold of the body of the lens and turn it towards the button. As you rotate the lens a white dot will come into view, there is also a white dot on the body of the camera. When it lines up with the white dot at the very base of the lens (not the one showing the focal length) the lens should just pull out. To put in the new lens hold down the botton, line up the dots, insert the lens and twist it into place.

Hope that helps,

Bart.