Hello. There is a procedure called a Knee Lift that can help smooth the upper knee area if severe excess skin is a problem. It does require an incision above both knees and a few weeks of recovery. A thigh lift addresses the skin in the upper thigh area, it will not help the knees alone. If the skin is not excessive, heating devices can help the area. Good Luck!
Usually saggy knees are due to loose skin which doesn't resolve with any amount of exercise. Depending on the severity you may be able to contract the skin with something like Forma Plus or BodyTite. If it's very significant you will need a thigh lift. The only way to know for sure is to book a consultation with a board certified plastic surgeon near you to see what your options are. Best of luck!