How would Trump re-negotiate NAFTA, "the worst trade deal ever"?

During his campaign, Donald Trump called NAFTA "one of the worst trade deals ever made." Yesterday, his staff said he was ready to pull America out. But after talking last night with Canada’s Prime Minister and Mexico’s President, today said, "So they asked me to renegotiate I will and I think we'll be successful in the renegotiation which frankly would be good because it would be simpler. But we have to make a deal that's fair for the United States ...they understand that." Shawn Donnan, world trade editor for the Financial Times, reports on Trump's change of heart.