To update your payment information (if you signed up for Avatar via our website):
Log into your Avatar account
Click on the gear icon in the upper right hand corner of the screen
Click on Settings
Select the Billing Info dropdown under the Account menu
Update your billing information
When finished, click the Update Billing button
If you signed up for an account via the App Store or Play Store, you will need to update your billing method through your Apple ID or Play Store account.