Vyřešeno:

DirectoryInfo d = new DirectoryInfo(Server.MapPath("/news/news"));
                    string d2 = Server.MapPath("/news/news/");
                    FileInfo[] f = d.GetFiles("*.txt");
                                         
                    for (int j = f.Length; j > 0; j--)
                    {
                      File.Move(d2+j.ToString()+".txt",d2+(j+1).ToString()+".txt");
                    }