Best exercise for flanks and upper arms

Unfortunately, you cannot tighten skin. It is what it is. As you lose weight, your skin will sag, but it will tighten over time naturally (depending on your age), but may never be as tight as when you were 20. I've had a tummy tuck and a brachioplasty to remove excess skin, and even that isn't perfect. You can build muscle to help fill in the space where the fat used to be, but that's about it. Skin is fairly elastic, however, so don't lose all hope since you're still fairly young.

#1 Keep eating in a calorie deficit in order to lose fat. This is the top way to tighten up appearances. Spot fat loss is not possible (meaning that you can't specifically choose to lose fat from one area or another.) As you burn fat, it will come off all over your body, albeit some places will lose more quickly/slowly than others and this can vary by individual. But the overall loss of fat is key.

#2 If you're not doing it already, start up a progressive resistance program. That means lifting weights or doing bodyweight exercises or something like TRX or a combination of those things. This will help you strengthen your muscles and retain muscle as you lose weight. Certain people build a bit of muscle while in a deficit but when your overall goal is fat loss and you're eating in a deficit, it's best to just aim to retain muscle. Even if you are not building muscle, you're going to end up more shapely once the fat is gone if you start resistance training now. It will make a big difference.

Good luck and happy recuperating

Your skin will tighten up as time goes on. It really does but it can take a couple of years after you have finished losing weight for it to get to its final "this is it" point. It's one more of those things where patience is a virtue. (Seems like nearly everything about the process of going from obese/overweight to fit falls under that umbrella at times.)