Go to the Form Your Team assignment, Click Edit, and changed it to a "manually formed team" in order for the gear icon to appear.
Now at this point, learners cannot form their own teams anymore, but you can create new teams, add them to existing ones, and in general move them around anywhere. Learners can continue to leave their own teams and request to join another one, if they wish.
You should now see the gear icon from the Assignment list page (near the bottom), go to "Edit Team", go to "Save and Edit Team" and it should take you to this screen to edit the teams. Note that existing teams are still in place.