There are a couple ways to fix it but warps in finished wings don't always come out permanently. Hopefully it's flexible enough that you can shrink the covering such that it removes the warp.

In the mean time a good method is to get two guys with hot gloves on. You each hold one end of the wing and move it back and forth over a pot of rapidly boiling water. Twist the warp out by going past straight in the opposite direction because the wing will spring back a little.

Hold the wing like that until it cools down. It takes a little time and sometimes a few attempts but that should get it pretty close.

Now that I think about it, are you sure the wing is warped? You used the built in rib tabs so maybe the "warp" is wash-out designed into the wing.