Multi-site: Change the URL of main site to HTTPS

I would suggest editing the wp-config.php file, and adding:

define('WP_HOME','https://example.com');
define('WP_SITEURL','https://example.com');

However for multisite here it says you should edit DOMAIN_CURRENT_SITE and PATH_CURRENT_SITE, if there is such thing defined in a multisite wp-config.

First suggestion is based on this article.