Nearly all of my fence was that way. Not my doing, it was in a really bad situation early in it's life. The best way to fix this causing the least amount of damage is using a small bottle jack. On the inside where the horizontal structure, place a 1 ft. piece of 2x4 in a horizontal direction under the top horizontal frame, a short piece of 2x4 cut the appropriate length in a vertical direction on the base horizontal frame, and the bottle jack in between. Jack it up just enough to let the skin go back in. While holding the skin in place, release the pressure. If the top horizontal structure is permanently bent upwards it too can be fixed but requires a little more trickery,