Should you wish to change your payment details for in App purchases you will need to update this on your Apple ID.
It is important to note that purchases done through the app go through iTunes, and are subject to Apple’s policies and practices. Zinio has no control over these:
Purchases done through the zinio.com can be made using paypal or credit card, and are processed by our payment processor and are managed/controlled by us. Those purchases can be refunded. You can change your billing information for Zinio.com purchases by going to Zinio.com > Account Settings.