Unregister a site as a hub site
If you're a global or SharePoint admin in Office 365, you can make a hub site no longer a hub site (unregister it as a hub site). Make sure you do this before you delete the hub site. When you demote a hub site, any sites still associated with the hub site will be disassociated the next time a user accesses them.
Unregister a hub site in the new SharePoint admin center
Some functionality is introduced gradually to organizations that have opted in to the Targeted release option in Office 365. This means that you may not yet see some features described in this article.
In the new SharePoint admin center, under Sites, click Active sites.
Select the site, click Hub site, and then click Unregister as hub site.
Unregister a hub site by using PowerShell
Download and install the latest SharePoint Online Management Shell. If you already have a previous version installed, uninstall it first and then install the latest version.
Connect the SharePoint Online Management Shell to SharePoint Online for your organization. For info, see Connect the SharePoint Online PowerShell.
Run the following command:
(Where URL is the URL of the site.)