Please watch the below video if you'd like to see how to do this (chapter 3).
Navigate to the Organisation & Units area.
Select the Unit you want to set the default Line Manager against and select Settings from the left-hand menu.
Click on the Change button on the right.
Search for the Line Manager by typing their name in to the Filter field on the right and click Search or filter by Unit.
Click on the radio button next to the name of the Line Manager.
Tick the option to Update all existing users if you you want to apply the default Line Manager to any Users already within the Unit.
Click on Finish.
The default Line Manger will now be displayed in the Settings tab information.
Any User accounts created in the Unit or moved to the Unit will now be automatically assigned the default Line Manager. In addition, if you ticked the 'Update all existing users' box shown above, any previous Line Manager(s) assigned to the User(s) will be replaced.