How Can Status Be Extended?
A Canadian citizen with TN nonimmigrant status may re-apply for an additional three-year period at a port of entry before the previous period of admission expires. The new application must include the same documentation and evidence as the initial application, along with an updated employment letter.
A Mexican citizen who wishes to extend his TN nonimmigrant status must return to the U.S. embassy or consulate and apply for a new visa. After the visa is issued, the applicant must present the visa at a U.S. port of entry, along with a valid passport and the required fee.
In the alternative, Canadian and Mexican citizens with TN nonimmigrant status may apply for an extension by requesting that their U.S. employers file Form I-129 with USCIS along with supporting documentation and the required fee.
How Many Times can a Mexican or Canadian Citizen Apply for TN status?
Can the TN Applicant's Family Accompany Him to the U.S.?
How Can an Applicant Apply for TN Status if Already in the U.S.?
H-1B v. TN - Weighing The Options