If your Mailsuite subscription has expired, you may find that the option to update your payment method is no longer available in your account settings. This is expected behaviour: once a subscription has lapsed, the payment update flow is no longer accessible.
To regain access to Mailsuite's paid features, you need to start a new subscription rather than updating an existing one. You can do this by visiting the Mailsuite website and subscribing again from the pricing page.