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.