Navigate to Organisation and Users via the Menu.
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.
Scroll down the page and click Move Unit.
Expand the organisation structure within the popup window.
Click on the Organisation Unit that you wish to be the new parent Unit, then click Submit.
You will be returned to the Organisation and Users page where you will see the structure has changed.