Navigate to the Menu and click on Organisation and Users.
Click on the + icon next to the organisation in which the Organisation Unit sits.
Expand again if necessary and then click on the Organisation Unit's name.
Click on the Details tab.
Click on Move Unit.
Click and expand the organisation structure within the popup window.
Click on the Organisation Unit that you wish to be the new parent unit.
Click on Submit.
You will be returned to the Organisation and Users page where you will see the structure has changed.