Move items from collection to collection

In the Library

You can choose media items from any of your collections or from your independent media Items.
Choose the collection in the upper left corner (little triangle after LUNA Library) 

Now you can select the media item(s) you want to move.
Highlight all the items you want to move together. 
In the More menu select "Add to collection".
From the drop-down select the collection you would like to add the media item(s) to.

See Also: How to delete a record or item