SharePoint Online has got a long awaited feature: you can now change/rename URL of a SharePoint Online site.
This capability applies to:
NOTE if you have enabled a retention policy on SharePoint sites, you will have to exclude (at least temporarily) the site you want to rename, otherwise you will get the error, with the Edit button greyed out
The address for this type of site can’t be changed
You cannot change the URL of your root site (https://<your tenant>.sharepoint.com).
A SharePoint (or global) administrator has the ability to change/rename the URL for a site from the new SharePoint administration portal (https://<your tenant>-admin.sharepoint.com). Unless you have change it, this should be the default SharePoint Online administration portal, otherwise you can switch by hitting the banner
From there, select the site you want to update the URL and access his properties
On the Edit blade shown on the right side, you will see the Edit option for the URL
When you hit the Edit option you will be asked to set the new URL; the system is automatically checking if the new URL is available
As you changed the URL, you are asked if you want to change the site name at the same time
Then the system is updating the site URL and generates redirects to redirect all access from the previous URL to the new one
Once the process is completed, the new URL is being displayed
The new URL is now active and the old one is being redirected to it automatically and transparently; there is no message displayed to the user.