Promote converts your subsite to a site collection by migrating it to the same level as your other site collections.
Note: You can keep your original subsite in the options when using Promote.
Index
Prerequisites
You have global admin, Farm admin, or SharePoint admin permissions.
You have site collection admin permissions on the site collection that contains the subsites you want to promote.
You have term store admin permissions on your source and destination if you have managed metadata (Terms, Term sets, Term set groups) in your migration scope.
Note: Site collection admin permissions are required even if you have higher admin privileges, such as SharePoint admin or global admin permissions. For more information, see Administrative permissions.
How-to
Select Copy from the sidebar.
Select the Restructure environment tab.
Select Promote subsite into site.
Click on the Tenant or web app at the top to select another one or to add a new connection.
Expand the site collection with the subsites you want to promote.
Add a checkmark to the left of the subsites you want to promote.
Click Continue to options.
Set your options.
Click Promote.
The operation will run, displaying the progress summary from the Promote report screen. You can access the summary and the Promote report anytime from Tasks.
Note: The permissions are handled as with Copy structure when promoting a subsite as a full site copy.