You can handle your account, subscriptions, licenses and payments from two places.

Inside the plugin

From the SEO Booster -> Account page you can access your details and billing information.

On wpsecurityninja.com

You can also go to cleverplugins.com/account/ to manage your account.

Here you can manage your sites, download the latest version of the plugin, find your license key, manage your payment details and cancel your subscription.