If you subscribed online to nkoda with your card:
Log into app.nkoda.com
Click on your profile icon, and then 'Account'
Scroll down the panel to the 'My Subscription' area, and click 'Manage'
Make changes to your card details
Alternatively, cancel your current subscription and begin a new subscription at the end of your next billing cycle, inputting your new card details. Don't worry, your library will still be there if you make a new subscription under the same account.
If you require any further changes to be made to your billing / payment details, please contact our support team by emailing support@nkoda.com.
If you subscribed to nkoda through your App Store:
You will need to change your payment details within the App Store itself.