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
Sign in to https://admin.microsoft.com as a global or SharePoint admin. (If you see a message that you don't have permission to access the page, you don't have Office 365 administrator permissions in your organization.)
In the left pane, under Admin centers, select SharePoint. (You might need to select Show all to see the list of admin centers.)
If the classic SharePoint admin center appears, select Open it now at the top of the page to open the new SharePoint admin center.
In the left pane of the new SharePoint admin center, select Active sites.
Select the site, select Hub, and then select Unregister as hub site.
If a hub site has associated sites then you unregister it, it might take a while for the sites to be disassociated. If you re-register the hub site, the sites may remain associated.
To learn how to use Microsoft PowerShell to manage and delete hub sites, see Manage SharePoint hub sites.