I have used this method which seems easy.
First put a padded clamp on the posts on either side of the bend. If there is a long space between posts add another clamp [or two] closer to the bend to isolate it. Put strips of cedar wood shingles on opposing sides and gently move each one toward the other,thin ends first, so that your secondary clamps, if used, are supported between the barrel and rib.
Finally , take another pair of shingles and wedge them together on each side at the center of the of the bend, starting with thin ends first. Tap them together from both sides until the rib raises. Go very slow and remove the shingle wedges under the bend often, looking to see how much more correction is needed. One does not want to bend it back too much.
