App Store can't update an app It just keeps giving the Update option, even after "updating" the app. Take a look at this video. Clicking the update button starts the process, but in the end, it reverts back to "update" instead of completing the process and becoming "open" 

How do I update the app ?
Delete and re-install the app. Unless the app in question is Xcode, I'd be doing the same for all apps, if update fails.