Remove a site's hub site status
If you're a global or SharePoint admin in Office 365, you can demote a hub site (remove its hub site status) by using Microsoft PowerShell. 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 automatically disassociated.
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.)