Yes, you can extend or change your US non-immigrant visa after entering the United States by applying with Form I-539, through the official USCIS (U.S. Citizenship and Immigration Services) website. However, you must file for the extension or visa change before the expiry of your authorized stay (I-94 record). You must also fulfill other criteria, such as obtaining a renewal or a new stamp, which will require you to exit the country. US visa extensions or changes are generally for temporary stays, which primarily require not violating any visa terms, with exceptions for the Visa Waiver Program.