Only Administrators are allowed to change a user's Role. You can change a Consumer-level user to an Admin-level user and vice versa. Please note that changing Consmer-level users to a power user (Admin or Contributor) will count against your combined power user seat count. 

See below on how to make these changes.

1) To change a user's user-level you will need to navigate to Settings in the upper-right hand corner where your profile picture is located:

2) Proceed to Users & Groups > Users > select the user you would like to change and select the edit icon:
3) Select the Role in the drop-down menu and click Save to finalize the change: