You can easily manage your Google Play subscription from the Viki app. Here's how:
While logged in to the Google account with which you purchased your Viki Pass subscription, log in on the Viki app to the account that has your Google Play Viki Pass subscription, go to Me > Settings > and scroll down to the Viki Pass section > Manage Subscriptions. You will see your subscription details and also the options to Cancel Subscription and Change Subscription
After making the desired changes to your account, you will receive an email confirmation from Google.