Unfortunately "bad knees" is at best a generic phrase which covers a wide spectrum of potential causes. Have your knees been assessed by a professional who gave you an recommendation as to what your problem is and what if any correction is possible? Without that type of information there is no legitimate response to your question.
No specific exercise is going to "tone and trim" your legs. Fat loss is an all over body process with your body picking where the loss comes from. You do not burn fat from the area you are working. Genetics largely determines where you lose fat from when. Your diet is actually the key to fat loss. No amount of squats, lunges, etc. is going to give you nice legs if you eat like crap. Maintain a caloric deficit. Do a good full body strength training routine 2-3 times a week and get in some cardio. If your knees are that bad, you should probably talk to your PT or doctor about what exercises would be safe for you to do.
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