2026 is the first mandatory review of CUSMA by the signatory nations. The trade agreement is currently set to expire in 2036. If there is a written agreement by all three parties to extend the treaty at the review the expiration date will be extended another 16 years to 2052.
If the three nations fail to agree in writing to extend the agreement in July 2026 then the expiration date of the agreement remains 2036. There will then be annual reviews thereafter where the parties have another opportunity to extend the agreement to 2052.
So, just because there is not agreement to extend the agreement to 2052 next July CUSMA will still remain in effect until 2036. IF the US (or Canada or Mexico) were to wish to unilaterally leave the agreement before 2036 they would have to indicate their withdrawal in writing and they would exit the agreement 6 months later. Apparently however there may be some statutory details in the enabling legislation in the US that may require Congressional approval to withdraw from the agreement.
There are lots of States that may not want to see the US unilaterally withdraw from CUSMA so Trump may want to walk a more narrow path by refusing to extend CUSMA next July and use the pressure of that to push for some modifications to the agreement to make it more favourable to the US in return for an extension.
Unilateral withdrawal by the US might cause more problems that would be worth it to the Trump administration...especially if the impacts of tariffs begin to bite in advance of the 2026 midterm elections.