Extending Your Stay
Persons admitted in transit (C visa) are not permitted to extend their stays in the United States. See Extend Your Stay on the U.S. Citizenship and Immigration Services (USCIS) website. You must depart the United States on or before the date indicated on your admission stamp or paper Form I-94.
Failure to depart the United States on time will result in you being out of status. Under U.S. law, visas of travelers who are out of status are automatically voided (section 222(g) of the Immigration and Nationality Act). If you had a multiple entry visa and it was voided due to you being out of status, it will not be valid for future entries into the United States.