This will depend on how the part was originally created. If it was created in NX sheet metal, then edit the feature (change the edge selected or reverse the length direction).
If not, make sure the part has been converted to a sheet metal part (using Convert To Sheet Metal command) and then use Resize Bend Angle command. Change the angle from 90deg to 270deg.
And if it was a Contour Flange... hold your breath because the following features will needs some serious reattaching...
and if it was a non 90° flange don't be surprised if the reverse solution doesn't look like something you initialy would expect.
But as Dave already mentioned the resize bend angle is capable of some cool stuff.