Complex sentence: The iPhone 7 and iPhone 7 Plus jointly serve as flagship devices of the iPhone series and successors to the iPhone 6S and iPhone 6S Plus; they are scheduled to be released on September 16,2016, with pre-orders beginning on September 9,2016.
Split into simpler sentences: The iPhone 7 and iPhone 7 Plus are the flagship iPhones, succeeding the iPhone 6S and iPhone 6S Plus. They are scheduled to be released on September 16,2016, with pre-orders beginning on September 9,2016.