The app kept on asking for update but when I click for update the app it is for open
The iOS App Store often has trouble showing the correct version of Castle Clash.
Enter the App Store, then go to the Updates tab.
Once at the Updates tab, "Swipe down" to force update the App Store. This should present the most up-to-date version of CC.
Why CC has this problem, and no other apps do, is currently a mystery.