You can easily manage your iOS subscription from the Viki app. Here's how:
Log in to the account that has an iTunes Viki Pass subscription, go to More > Settings > and scroll down to the Viki Pass section. You can see your Current Plan, Change and Cancel your subscription.
Any action you perform from the app will take you to iTunes Store subscription section, where you'll be able to change plans, cancel your account and see your subscription details, directly from your iTunes account.