Nafta came into force in 1994. It was passed into law by the respective Parliaments of the three countries: US, Mexico and Canada.
Donald Trump says he wants to change it to provide more benefits to the US . The Agreement has a provision that amendments are possible if agreed by the parties to the Agreement:
Article 2202: Amendments
The Parties may agree on any modification of or addition to this Agreement.
When so agreed, and approved in accordance with the applicable legal procedures of
each Party, a modification or addition shall constitute an integral part of this Agreement.
So the three parties would have to agree to any amendment and passed by their respective parliaments . This will not be easy, I suggest, and will take time.
A party to the Agreement can withdraw:
Article 2205: Withdrawal
A Party may withdraw from this Agreement six months after it provides written notice of withdrawal to the other Parties. If a Party withdraws, the Agreement shall remain in force for the remaining Parties.