The hierarchy of a collection can be changed from the 'Edit' page of that particular collection.

You will be directed to an edit collections page after you create a collection or when you click on a collection from your institution's collection tree structure from the collections option.

 The Edit Collection Page as seen from the figure above allows a user to edit properties of an existing collection. 

Changing the Collection Hierarchy:

You can also change the parent collection under which this current collection appears. Changing the parent is simple, simply click on the 'Edit' button under the current Parent collection name and select the new Parent collection. The current parent is shown in red.
Simply click on a new parent collection, If you wish to make this collection as a top level collection under your institution simply click on the very first Collection also known as the root collection.
Your changes are saved automatically.
Please note that all sub-collections under the collection you are trying to move are also moved automatically to the new parent.