Giving Multisite its own directory with root domain

I have a WordPress Multisite, installed in a directory, that needs to respond to the main domain. There is a guide for this operation, which cites, but is not explicit as to what should be done in this case:

Giving WordPress Its Own Directory

I was able to do this when the site was standalone and it works fine, however Multisite does not allow to directly change the Site URL in the administrative (network) area of ??the main site. I assume you need to change the database. What I failed to succeed.

Any suggestions where I should look? I appreciate any help.