If can be as simple as pointing a blow dryer at the control surface and applying pressure to the thing to twist opposite the warp. When it cools, release tension and check if you have corrected it.

Note... Mnokote takes a "set" if ypu heat it to JUST below meltng point. This makes it resistant to sagging or allowing the warp to return if you heat it to the correct temperature while removing the warp. You can then leave the arplane in a car in the sun and the cover won't sag.