This feature is available to Team Owner and Admins only.
Select the team for which you want to see the members. You can do this by clicking the drop-down menu under the account name which will display all your teams.
Click the Users button on the left pane. Inside the Users tab, you will see a list of all the team members. Click the edit icon (pencil) on the right hand side of the team member whose role you want to edit.
From the resulting pop-up menu, you can change the role to admin/member.