What workouts help you gain a bigger butt & thighs instead of losing weight?

So I’m already a small girl, I been eating a lot and gaining the weight I been dying to have, currently I’m about 121 and I want to gain to 140, but I also really want to work out my butt and thighs to make them bigger then what they currently are right now

What workouts can I do or what app is good for this?