You can rename a SharePoint Online site via the admin portal, but you must remember the following additional activities:
1. Rename any Groups or Teams that are associated with the SharePoint site - do this via the appropriate O365 Groups / Teams admin portal(s)
2. SharePoint will create a redirect on the URL i.e. siteA is renamed to siteB. But SiteA URL will still work because a redirect is created behind the scenes.
This may prevent the site name from being re-used. So if you want to remove the redirect follow these instructions : https://docs.microsoft.com/en-us/sharepoint/manage-site-redirects
*to connect to our (SMU) tenant use the following url, not the 'contoso' ones in Microsoft's documentation:
https://smuhalifax-admin.sharepoint.com