Give yourself time, you are still growing and everyone goes through that awkward stage in the middle, where things seem to be disproportional to each other. Eventually everything will work itself out fine. If you are concerned you can always start exercising to try to build up some muscle, but make sure you don't over do it.
You can't fix it, because theres nothing wrong with it. A teen goes through that stage. You just don't notice it unless it's you. Remember when your baby teeth came out, and your permanent ones came in? If you don't, then look at a cousin or sibling, and you will see that when the teeth come in they are too big for your head or your size. In time everything catches up and turns out perfect.