I have a several times repaired Shark 402. With the last repair, the inside wing hangs down in the back, causing the outside wing to hang down in flight.
As Dan notes, it depends on what it is covered with. Heat-shrink, twist it past straight, then hit it with a heat gun until the wrinkles are gone. Keep adjusting until straight. Don't expect it to stay for long, at least the first time you do it.
Doped or painted, steam in extremely large quantities might work, but for something really sturdy like a silkspan d-tube, wrap it with a bath towel, boil up a big pot of water, then while you twist, get someone to pour the boiling water on the towel. You will almost immediately feel it release and move, leave it for a 10 seconds or so after that, then have your helper unwrap it and let it cool down, so it stiffens back up. This is pretty much the only thing that will fix full-sheeted or D-tube wings with dope or other paint. This method is reasonably stable on built-up structures, even fuselages.
BTW, while I know it doesn't apply here, none of this will work at all on sheeted foam wings. But of course, once you sheet them, they aren't prone to warping with time, no matter what you put on them. If it is built warped, "fixing it" is a relative term, and any reliable method requires extreme measures involving large Exacto knife cuts and epoxy. Or another core and some more wood.
Brett