Use Promote to upgrade your sites as site collections.
- You are connected as a Farm Administrator or Tenant Administrator.
- You are connected to your Admin center or Central admin.
- Click Restructure .
- Click Promote subsites .
- Click on the web app at the top to select another one, or to add a connection.
- Expand the site collection with the subsite(s) you want to promote.
- Add a check mark to the subsite(s).
- Click Continue to options .
- Adjust your options.
- Click Promote .
- The task will run, and you will see the promote summary screen (you can access the summary anytime through Tasks).