This may be caused by a few things.
Firstly, make sure that your internet connection is stable and app updates are not blocked because you’re on 4G/LTE instead of Wi-Fi. Secondly, make sure that your device has enough storage space to download the update.
Another possible cause is that the app might be pending in the queue (the most common issue on Android devices). Follow the steps below to clear the downloading queue:
1. Open the Play Store app.
2. Swipe-in from the right edge of the screen, and select My apps & games.
3. Here you’ll see all apps that are currently downloading on your phone. To cancel an app from downloading, select it and touch the X (cross icon) near the download progress bar to cancel download for that app. You can do this for all apps to clear the download queue.
After that, try to update the game again.