Try lightly wetting the balsa on the bottom of the wing by spritzing it with Windex or water. Bend it the opposite way (up in this case) that it's warped a bit further than the correct position. Then, boil all of the water out of the balsa with a very hot covering iron while it is held in this position. If that doesn't fix the warp, repeat or do the other side too. Good luck and let us know how it turns out.