If your six iron is only going 140 yards, and you're average sized (and not a teenager), spend your money on some lessons first. You're missing distance due to poor mechanics.

That said, if you're smaller than average, young, or a woman, then a reshaft with graphite might work. You'll probably realize a 5-8 yard increase, assuming that all things remain equal (and you may need to get your heads bent to account for the extra length).