When you entered the country as a nonimmigrant, a U.S. immigration inspector should have examined your passport and visa and then give you (or placed in your passport) an USCIS Form I-94 (Arrival/Departure Record). It is this record (not your visa) that tells you, in the lower right-hand corner, when you must leave the United States. If you want to extend your stay in the United States, then you must ask for permission from the U.S. Citizenship and Immigration Services (USCIS) before your authorized stay expires.
Documents
1. Form I-539 (Extension of stay/change of status)
2. Fee for filing extension: $200
3. The photocopy of I-94
4. Copies of all pages of your passport
5. Proof of financial support
6. A copy of your return air tickets (this would be helpful to show that the stay is temporary)
7. A letter regarding the reason for extension
After approval
If you are approved, you can simply stay until the date you are granted extension. You will get new I-94 card in the approval letter. When you leave U.S., give both I-94 cards to the airline staff when you check in at the airport. You should save the approval letter, boarding passes and copies of airline tickets to prove that you left on time and you were not staying in unlawfully.
What if denied?
If your extension is rejected, you have to leave immediately as you get out of status immediately. Also, if you previously had multiple entry visa, that visa is void now and you will have to apply all over again at the consulate when you want to visit next time. And you can not appeal this rejection decision.
Following documents need to be sent to USCIS after you exit the country after an extension denial:
1. Original denial letter
2. Original boarding pass
3. Copy of biographical page of passport
4. Copy of entry stamp in new country
5. Letter saying that applicant overstayed because he/she was awaiting decision on extension application