It depends on the cause of the wing being warped. The picture you showed looks more like an issue with applying the tissue paper than a warped wing to me, but pictures don't always clearly show small warping or other issues. If it's an issue with the tissue, you can either cover over it with another small piece or more optimally, remove the tissue and re-cover. Cut off the smallest amount of tissue possible with a sharp razor blade and then patch over it with a new piece using a glue and water mixture to blend the edges. If you do it carefully you frequently cannot even see where it was done.
If the structure of the wing is actually warped, the most likely cause is also the tissue paper. When you shrink the tissue paper to the wing, it frequently will bend the wing a little bit. If you shrink the tissue paper on the opposite side from the direction the wing is bent, it can sometimes reduce or eliminate warping, but you need to be careful not to shrink it too much or you can tear the tissue paper or break the balsa. If that doesn't work or the tissue paper is already sealed, the next step is removing the covering material and seeing if the wing remains warped. If the wing is no longer warped without the covering material, it can be safely re-covered with care being taken to shrink tissue evenly across the whole wing. If the wing remains warped, give it a couple hours to see if it slowly recovers. If it doesn't and the type of cement used allows it, you can try soaking the wing in water (or rubbing alcohol or window cleaner) and very carefully bending and clamping the wing into the correct shape until it is fully dry. If even this does not work, you're pretty much stuck rebuilding the wing.
Most likely the issue is caused by the covering tissue and can easily be resolved.