You mean extension of Visa itself or Extension of Authorized Duration of Stay? Visa Cannot be extended within the US. If it has expired, it must be obtained from ourside the US (for B1/B2, I think it must be one's home country ... I am not sure about this). Extension of Duration of Stay is simple process. Just need to speciry "why". If reason for extension is good, there is no problem with it. Denial of extension doesn't by itself cancel the current visa.