Changes the stacking order by moving the selected object behind an object that you specify. The screen location of the selected object does not change.
Choose Shape - Arrange - Behind Object (Office Draw only)
Open the context menu of a selected object and choose Arrange - Behind Object
On the Drawing bar, open the Arrange toolbar and click:
Behind Object
Select the object(s) that you want to move behind another object. Right-click and choose Arrange - Behind Object, and then click an object in your slide.
Arranging objects affects the stacking order of all objects in your document.