If you want to change the credit card associated with any Facebook page you manage with ManyChat, here's a quick and easy guide for you:
1) Choose the page where you want to update your billing info;
2) Head to Settings:
3) Navigate to the 'Billing' tab:
4) Click "Update Billing Credentials" button and fill the form:
You can also click "Remember me" if you'd like to store your payment info for quick checkouts later. Here's more information on how it works: https://stripe.com/checkout/info
5) After you finish filling the form, click "Update Card Details" button.
That's it - hopefully this helps!