If the department heads are wrong, you can change the department heads by going to the Directory tab > Department Chart > Edit (in between the "X" and "Delete" > change the name of the department head > Save. The department heads have no correlation with a employee's manager. The Department Chart nests employees under the appropriate departments and not under their managers.
To see the reporting structure, you will need to view "Reporting Chart" within the Directory tab. This will display which employees are reporting to which managers in TriNet Perform. Or, you can click on the pencil icon , in the Directory tab, for each employee to view the employee's manager within the "Edit Profile" window.