how to "switch around" proportions?

^you cannot spot reduce. you grow as a whole or shrink on the whole. certain areas store fat longer, but it is what it is.

Most guys need bigger lower bodies.

To be honest, at 6'1", I don't think you want to lose thigh mass. I don't think you want to risk getting "chicken legs."

I think it really depends on your bone/muscles.

It's true that there are a lot of guys at the gym (curl jockeys) who have built big upper bodies but still have undeveloped legs, but among men who have never worked out much, I think there are plenty who have disproportionately scrawny upper bodies, and the emphasis on squats as the foundation of any workout program, common among many weightlifters today, seems misplaced for these guys. I also think the "huge legs" aesthetic valued among contemporary bodybuilders looks pretty odd to the general population; paintings and sculptures suggest that it's a historically unusual ideal, as well. To the OP: You won't have success slimming your thighs unless you lose weight all over your body, and you'll end up looking even worse in that case. I'd suggest instead focusing on building some upper-body mass to balance out your lower body. Any additional weight you add to your body will probably result in a bit more thigh muscle (since your legs power you around in all your daily activities), but you'll end up more proportionate overall if you add some size to your back, shoulders, etc. (Increasing your shoulder-to-hip ratio, in particular, will help you look better in clothes.) I'm not saying to neglect lower-body strength altogether, but just to skew your workouts toward the upper body, compared to some of the squat/deadlift heavy programs frequently recommended to lifting newbies, until your proportions look more normal. Ordinary weightlifting is not going to screw up your growth. And at 6'1", you don't have anything to worry about in the height department anyway If I were you, I'd try to take advantage of your youthful hormones and add a good 20 lbs of lean muscle to your frame over the next couple of years. You probably look normal among your peers right now, but by the time your reach college, you may feel comparatively tiny with 150 lbs on a 6'1" frame.

Start working out, doing higher weight/fewer reps for your upper body, and lower weight/higher reps for your legs.

And, at 6'1 150lbs, I don't think you should be trying to lose weight any where. I'm 5' 9" 155 lbs and would consider myself skinny (though, on the upper bounds of skinniness).

You're still growing, you're 15! Don't worry too much about it and just live your life, you can't be 15 forever. And if that's mostly muscle you're talking on your legs then you're in for a treat. There was this kid that used to be called bubble but by their friends when they were little. His father used to say "they'll be jealous some day". You have no idea how much women look at men's butts, more probably than men watch women