When I was a teenage I remember girls doing exercises to try and increase boob size. The truth of the matter is that the breasts arent a muscle so you cant increase their size. Actually, they are made of two tissues: fat and breast glandular tissue. If you lose fat then you will lose some boobs. But what you can do is work the chest muscles with your exercise routines to help give a natural lift so to say.
You wont increase their size but they will look a bit higher than before. And youll feel better because you will lose body fat from all over your body too. Good exercises include bench press, push-ups, and dumbbell pull-overs.