How to change holiday approver for an employee

Log in as an Administrator
Select the Directory from the black ribbon.
Select the office you wish to search on (Head Office in this instance).
Select View for required Employee.
Select Settings from the left-hand side menu.
Change Holiday approved by field to chosen new manager
Click submit to save this change.
Note - Holidays can only be approved by a Manager or an Administrator. Make sure that the manager has Time Off permissions switched on.
A guide to this can be at Manager Permissions Explained