Here's an exchange between David Sanger of the New York Times and Donald Trump:
SANGER: You’ve talked about building the wall of course. Would you amend or change Nafta?
TRUMP: Oh, without question.
SANGER: Tell us how.
TRUMP: Without question. Nafta ——
SANGER: Would you pull out of Nafta?
TRUMP: If I don’t get a change, I would pull out of Nafta in a split second. Nafta is signed by Bill Clinton, perhaps the worst trade deal ever signed in the history of this country. It’s the worst trade deal ever signed in the history of this country and one of the worst trade deals ever signed anywhere in the world. Nafta is a disaster. You have to understand, I just campaigned, as you probably read, and I won all these states, and one of the reasons was because of Nafta. Because Nafta has drained manufacturing out of New York State, out of Pennsylvania, out of Ohio, out of so many different places. It’s drained. And these companies have gone to Mexico, and they’ve gone, they’ve left with the jobs. David, I have statisticians, and I know, like if I went to Pennsylvania, I say, “Give me the statistics on what is going on with respect to manufacturing.” Numbers — 45, 55, 65, I have states that are so bad. New England. Look at New England, what happened. Nafta has been a disaster for this country. And a disaster for the worker and Nafta is one of the reasons that, you know, there are people that haven’t had a wage increase 18 years in real wages. Actually, they’re lower, some are working two jobs, working much harder, then making less and they’re older. It’s supposed to work the opposite. You’re making more, you’re making more I hope.
HABERMAN: What kind of change could you make in terms of Nafta without fully withdrawing from it? How could you?
TRUMP: You’ve got to be fair to the country. Everyone is leaving. Carrier just announced they’re leaving. Ford is building a massive plant. So I have a friend who builds plants and then I have to go. I have a friend who builds plants, that’s what he does, he’s the biggest in the world, he builds plants like automobile plants, computer plants, that’s all he does. He doesn’t build apartments, he doesn’t build office space, he builds plants. I said to him the other day, “How are you doing?” He goes, “Unbelievable.” Oh, great, that’s good, thinking about the United States, right, because he’s based in the United States. So I said, “Good, so the country is doing well.” He said, “No, no, not our country, you’ve got to see what I’m doing in Mexico.” He said: “The business there is unbelievable, the new plants we are building. People moving from the United States.” That’s what he does. One-story plants. You understand?
Sanger asked some very clear questions about how Trump would change or amend NAFTA, and as you can see from the rambling answers, Trump doesn't have anything specific to offer in response. That may be because, in order to respond, he would need to have some idea of what's in NAFTA, and it's not at all clear that he does.