if (FileUpload1.HasFile)
{
str = MapPath("~/Uploads") + "\\" + System.DateTime.Now.ToString("yyyyMMddhhmmss") + FileUpload1.FileName;
FileUpload1.PostedFile.SaveAs(str);
str = "~/Uploads/" + System.DateTime.Now.ToString("yyyyMMddhhmmss") + FileUpload1.FileName;
}
here i append timestamp along with file name , like this u can change filename