While there are certainly leg exercises to strengthen the muscles around the knees (quadriceps and hamstrings),you will need a combination of good, healthy nutrition and exercise to take care of reducing the fat. If you have seen an increase in fat around your knees, the first step would be to take a look at your diet. Journal your daily calories and see if you are staying within your allotted calories for the day—and if you are eating the proper kinds of foods. If you think you need help with this, a Registered Dietician can help you.
Second, you need to add a mix of cardio and strength training to your routine if you haven’t done this already. This will help you to burn calories and build muscle. A personal trainer can help you get a program started if you do not have one currently.
It is the combination of a healthy diet, strength training, and some cardio that will help you reduce the fat around your knees.