This particular image can not be fixed.
Its been "posterized" -- meaning there are only three different colors of pixels in the entire image: yellow, light brown, and dark brown (actually, there are more colors in the image, but but only due to the jpeg compression and still not enough for any level of detail). All the other details have been thrown away when it was edited.
Without either the original photo, or the original untouched scan, there's nothing that can be done.
1st law of image editing -- you can remove pixels (details), but you can never add them.