If you wish to remove a role from an admin, press the three dots next to admins name in the overview, then X next to the admin role. If the admin only have one role, you need to delete the admin by pressing the three dots.
You can not delete yourself as the administrator, another administrator has to do so.
You can not delete an administrator that is the contact person of the group. The contact person has to be changed to another first. Read more.