You can manually upgrade a subsite to a site collection using the Copy structure and content option in Migration.
For information on how to convert your subsite to a site collection using the Promote feature, see Walkthrough - 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 like SharePoint admin or global admin permissions. For more information, see Microsoft administrative permissions.
How-to
Select Copy from the menu.
Select Copy structure and content.
Enter the site address of your source. You can connect using a site address, SharePoint admin center address, or central admin address. Review our site address guide for connection article for more information.
Add your credentials, then click Connect.
Select your source. You can select a single subsite or a site collection if you want to upgrade multiple subsites located under it.
Click Next.
Enter the site address of your destination. You can connect using a site address, SharePoint admin center address, or central admin address. Review our site address guide for connection article for more information.
Add your credentials, then click Connect.
Select your destination.
Click Next.
Locate and select the source subsite you would like to promote.
Locate and select the site level in the destination.
If you need to apply specific parameters, select your source and update any Options, map users and groups from Mappings, set permissions levels, or implement site templates.
To perform the migration:
If you are connected to the central admin at the destination - With the source subsite you want to promote selected, click Start copy to migrate the subsite as a site collection.
If you want to merge into a pre-existing site collection - With the source subsite you want to promote selected, select Merge with X in the Options, then click Start copy to migrate.
Once the action is complete, the migration report will show any successes, warnings, or errors. See Migration report for more details.