While I understand the dissatisfaction with such changes in our bodies, I would caution against losing sight of what is important. If you do not have knee pain, proceed cautiously with increasing your exercise volume for you legs. The other answers already hit upon things like nutrition for losing fat weight, but you cannot target the area of the body where such weight will be lost.
Whether you have leg/knee pain or not, I would also suggest that you seek medical clearance to exercise if you haven’t already. Lastly, if you have seen a significant change in the shape of your knees/knee joints, you should also discuss this with your physician.