Apple forbids even mentioning alternative payment methods within the app.
> However, the rulings established that Apple's so-called "anti-steering" rules—language prohibiting developers from mentioning cheaper or alternative purchasing options that might be available outside of an app—were anticompetitive.
> Apple has updated its App Store rules to allow developers to provide external links to other payment options, technically circumventing its normal fee structure.
After that you can maintain your account via the web.