When you enter the game, and you see that there is an update, if you tap on the update button, it will lead you to the App Store.
But in the App Store, you only see the [Open] button, instead of the [Update] button, try the following:

1. Go to [App Store] and tap on the [Updates] button in the buttom of the screen.
2. Refresh the [Updates] page by pulling the screen down.
3. In the refreshed [Updates] page, scroll down to find PokoPoko and start the update.
4. If you still cannot find PokoPoko in the [Updates] page after refreshing it, turn off your device completely and turn it back on.
Then go to the App Store to check for the updates.