Site Administrators can add department administrators in two ways: 

One: from the Organization Chart which can be accessed through My Workspace; 

Two: from the Departments section which can be accessed through Settings. On the Organization Chart, go to Departments. Here is a list of all the departments in the organization that are available.

When you click on a department, it expands and you can see all the users who are part of the department. To make a user a department administrator, just click on the ‘Mark as dept. Administrator’ icon.

Navigate to Setting -->> Departments