This is the first time I've heard someone wants to lose both fat and muscle mass.
Of course there's a method to lose both fat and muscle: to be on a hypocaloric and low-protein diet (Advanced Nutrition, 2017). Such diet would very likely reduce fat and muscle in your body in general, not just in legs.
You can spot-reduce muscle mass in your thighs by using your legs less. Or you can try different trousers. It is said that clothes with vertical lines make you look taller and slimmer.
Spot reduction of body fat in your context does not make sense. Even if spot reduction of fat works, by training your thighs, you would lose fat but gain muscle in your thighs.
Fat distribution is genetically determined (Science Daily, 2010), so you lose fat from where you have it. This article about spot reduction of body fat mentioned in the comments does not say what was the original fat distribution in 16 participants (all women, by the way).