All payments and invoices to purchases to Nelio Content Premium are effected and managed by FastSpring (the secure payment platform we use to manage purchases and renewals of our products). To change your payment details, look for the purchase confirmation and receipt email you received when you made your purchase. Once you have it, click on the link Manage Your Orders.

For security reason, you’ll first be requested to enter the email address with which you subscribed to Nelio Content. Enter it and you’ll have access to your FastSpring account.
In the Orders tab you’ll see the details of your order; in the Subscriptions tab you’ll see all your subscription information; and the Account Details and Payment Methods tab shows the details of the account and payment methods.
From the Account Details and Payment Methods tab, you can add a new payment method (Add Payment Method) by entering the required information to change it. FastSpring will automatically use the new payment method at the next renewal.
How to access your FastSpring account directly from Nelio Content in your WordPress
Alternatively, you can go to the WordPress installation where you have Nelio Content installed and access the Nelio Content » Account menu. There, you will see relevant information of your account:

You can see the details of the plan you have subscribed to, the amount and date of your next charge, the email address with which you made the purchase, the license code that we sent you by email and the billing history to date with links to your invoices. You also have a link, Manage payments, with which you can access your payment methods to manage any changes.