The easiest way to convert a single Team from unmanaged to managed is to utilize the Admin Dashboard.
Navigate to Teams > Admin view.
Select Unmanaged from the Space list up top; it might be under the ⋯ button at the end of the list, depending on your screen size.
Find the Space you want to convert under the Unmanaged space list.
Click on ⋯ at the end of the corresponding entry on the list.
Select Convert to managed team.
Choose the desired Template from the list.
Click Next and edit Team properties as necessary.
Click Convert to managed team to finalize the procedure.