Cardio to reduce the size, running, walking or aerobics. Then light resistance training. Buy yourself some 2kg hand weights and do some squats and lunges every other day. You need to use the light weights to get the definition in your legs. Have a look online, lots of squat and lunge exercises you can do.
Absolutely, you don't have to do a really deep squat, but with the light weight it will challenge you and will sculpt your legs really well. If you look on Pinterest there are lots of things there to try and some specific for weak knees