New iOS 8 and iOS 9 for old iPhone, iPad model getting stuck app during download/ Update or install from app store. On my personal experience and my readers experience on stuck app on iPhone/ iPad, I recommended possible solved solution for your iPhone 6S, iPhone 6S Plus, iPhone 6/ 6 Plus, iPhone 5S/5, iPhone 4S/4.
Possible fix and solved solution on Stuck app on iPhone, iPad or iPod Touch
Note: Low internet speed one of the most hesitating reason behind stop app download from app store. So measure Mobile data/ WiFi internet speed on Speed test checker app. Available for all Apple devices and Platforms. (I am not talking about WiFi signal strength, Sometime full signal strength lose data packet).
Not enough storage space, Means free space should be greater than your app size. Navigate to check current free space Settings > General > Storage & iCloud Usage > Under Storage section (Available storage is a free space for install app or save files)
Learn how to free up space on iPhone, iPad and iPod Touch.
Once Reboot your device by restart or force restart iPhone, iPad and iPod Touch. (Press and Hold sleep/ Wake button for restart or Press and hold Sleep/ Wake button + Home button at once)
Might be apple server connection fail, you need to be a re-login for create a new login session. So, Sign out from your device completely, Settings > iTunes & App Stores > Tap on Apple ID > Sign Out
Done your job through iTunes installed on Mac/ PC. Connect your iOS device with PC via lightning cable and From iTunes > App Store > Remove stuck app or waiting app from there > install from there and sync with iPhone/iPad/ iPod from iTunes button.
Above solutions are the most helpful on Stuck app on iPhone, iPad or iPod Touch running on iOS 8, iOS 9 or Other.