Unfortunately for network versions of WordPress I couldn’t find an easy “click this”, “click that” way to change WordPress Multisite Network from HTTP to HTTPS for the main site (site 1)!
To change Site 1 (the main site) from http to https go to the hidden WordPress Options page https://domain.tld/wp-admin/options.php and change the two options “home” and “siteurl” to the https version.
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