If these are intrinsic stains (due to fluorosis etc.) then these can not be removed only reduced by some special tubes used for bleaching. Sometimes a layer of tooth coloured material may be put on front surfaces which are called as laminates but all depends upon case to case. May be it is not possible in your case. You may consult a prosthodontist or an endodontist (superspeciality in Dentistry) for the same.