Open the Filter menu, and then select “Noise” and “Despeckle.” Photoshop automatically applies despeckle algorithm to the image, which detect the edge of an image and blurs all but the edge of the image. If you find that the blurring effect is too great, go to the Edit menu and select the option “Fade Despeckle.” Choose the amount of despeckle to remove by entering a value in the Opacity box. The higher the number in the box, the more of the image reverts to its original state.
the whole article is as step-by-step as this excerpt. well worth saving.