If it hasn't been covered, try steaming it over a pot of boiling water, and letting it dry while weighted to twist it a little farther than you want it to be. Or dampen with Windex that has a little ammonia in it, and let it dry the same way. You might need to loosen the glue joints between the ribs and spars first. Wet and twist, let dry and reglue. If it's covered, twist and reshrink the covering. An extra set of hands and some cotton gloves are very helpful. Don't expect a one-shot success. You may have to mess with it and re-do it a few times. But usually you can get one acceptably straight
Kelvin.