Hi Andrea!
Unfortunately you can’t change the shape of the breasts themselves. What you can do is firm up your pectoral muscles, which can create the perception of a tighter chest area. Push ups with your arms very wide apart and bending outwards, chest flies where you lie on your back with your arms spread wide, then draw them together holding dumbbells, and ball squeezes – where you hold a small inflated ball in front of you between both elbows and SQUEEZE, will all contribute to a tighter chest.