What form should I file to extend TN visa?

For TN extension, you can either go outside the US and cross back over with the adjudicator being Customs and Border Protection or you can request USCIS for an extension in the US. You would use form I-129 with the TN supplement. The address to mail to USCIS is: USPS, FedEx, UPS, and DHL deliveries: USCIS Vermont Service Center Attn: I-129 TN 38 River Road Essex Junction, VT  05479-0001 I-129/I-907 Premium Processing Premium Processing Service USCIS Vermont Service Center Attn: I-129 TN 30 River Road Essex Junction, VT 05452-3808 Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.