For anyone wondering, the answer is that exercise won't change a concave sternum, but you can make it look a little less odd with normal chest exercises (pushup, bench press, etc). If you're really thin, it can also distort lower on your body, so upper abdominal exercise (sit-ups, planks, leg lifts) can help, as well as obliques (crossing or side variants of the previous group).

I suppose that the pic is from 4chan and the answer seems pretty ableistic and disrespectful for me, I do not expect more from this site.