Currently there isn't any option under WooCommerce > Settings > Integration > ShipStation to regenerate or change the key.

This allows vendors that are connected to remain connected and it's not possible to disconnect them or regenerate the key in the front-end.

You can do it yourself in the database in wp_options in the woocommerce_shipstation_auth_key option but there should be an easier front end to change it and revoke any ShipStation account that is already connected.