The Senate votes to modify or repeal it, and the President signs off. Same with any time the US does anything with a treaty.
Difficulty: Being a part of the WTO has, on balance, been extremely beneficial to the US, even accounting for this ruling. Seriously - the US has been a party to more winning cases in front of the WTO than any other country, by far. Should we just poop all over every other industry in order to maintain some sort of neo-Puritan approach to gambling?