Configure SSL on the main site of the multi site network.

I need to configure SSL on the main my network site.
WordPress does not allow me to change the website address to https: //.
When I click on edit the main site I am not able to switch to https: //.
In all other network site I am able to change, however, the main site no.

Please see the attached image.

How do I change the WordPress settings to use https: //.
Your hired and installed SSL on my server.