If you need to update the information for your monthly Leanpub subscription payment, please go here https://leanpub.com/author_dashboard/subscriptions and switch to a Free plan.
Then, you can switch back to your Standard or Pro plan, and enter your updated payment information.
Please note that the Free plan lets you have up to 100 books and courses, so all your content will be safe when you make the transition. Also, any custom Book Theme settings you have set up will not be lost while you do this.
We're sorry for this clunky "just unplug it and plug it back in" method for updating your subscription information; this is something we intend to improve in the future.