First of all, check that you have enough free memory on your device or free up some space if needed. Then fully close the game: first press the HOME button once to minimize the app, then press the HOME button twice to see all active apps, then tap and hold on the game’s tab until it starts vibrating, and then select the “-” icon to close the game. Then press the HOME button once again to hide the panel. If you still can’t download an update after these steps, contact Apple Support by following this link: