To change a business unit's team lead, click on the business unit's name.

In the Business Unit dialog box below, you will see your users belonging to your business unit and the selected team lead (marked with a green tick).

Click on the Assign Team Lead button:

The Assign Team Lead dialog box appears. In the Select Team Lead drop-down list, you can select a different team member to become the team lead or you an select the empty value for no team lead and click Save. You will see the change in team lead confirmed in the Business Unit dialog box:

Click the x button to close the dialog box and return to the business units structure.