If your credit card is expired or having issues and you need to update your credit card information, here is how:

Go to the Monto billing page:

If your card wasn't able to be charged, you'll see a notice like this on your active apps that failed:

Click the "Manage Subscription" button.

Then click the "Update Payment Method" button at the bottom of the popup.
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This will open up a Stripe panel where you can update your credit card information.