1. Open the Settings app.
   
2. Scroll down and tap on iTunes & App Store.
   
3. Tap on your Apple ID (an email address) and tap Sign Out (note that you will need your password in step 6 to sign back in).
   
4. Turn off Wifi and cellular connectivity by enabling Airplane Mode.
   
5. Reboot your device as described here. Note that this procedure is NOT the same as merely powering your device off and on.
6. Disable Airplane Mode.
   
7. Head to Settings > iTunes & App Store and sign in with your Apple ID.
   
8. See if the download of the app resumes correctly. If not, open the App Store, search for the app in question and tap on the stop button. Wait a couple of minutes and then tap on it again to resume the download.
   

Problems installing or updating are unrelated to the app itself. If you continue to have problems after following those steps, please contact Apple Support at:

https://getsupport.apple.com