You can change your payment method by logging into your account, going to Billing, and clicking on “Manage Billing”.

Once you click the button, you’ll be taken to a separate page containing information about your subscriptions and payment methods.
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If you are a Stripe user, click on “Add payment method”:

Enter your debit/credit card information:

To change your default payment method, select “Use as default payment method”.
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If you are a FastSpring user, click on Manage >> Update payment method:

Then, add your new payment method: