So far, the only way to replace the root site in a SharePoint Online (SPO) tenant was through the Invoke-SPOSiteSwap cmdlet. Luckily, with the latest features added to the modern SPO Admin Center, we can also make that site swap by a very simple side panel were we only need to type the URL of the site that is going to replace the root site. One important thing to note is that the replacement site cannot be a Hub Site or Group site (connected to an Office 365 Group):
Note also that in the Replace root site pane you can see the root site is going to be archived in the tenant. Once we start the replacement action, it will take some minutes until the new root site is available and the old root site is archived.