Moving Wordpress multisite from folder to root


I have the website '' installed on its own folder.
However, today I activated Multisite, and created a subdomain. On the way, I found out that Multisite doesn't work if the main install is not in the root.

I would like to safely move the wordpress installation of '' in the root, then continue with the multisite.

Could you please give me some advice?
Notes: There is one subdomain created, but not really important, I also want to get rid of it.

Thank you!