You may realize a file doesn't fit well in a specific collection so you can remove it.
Click on the collection you wish to remove the file(s) from.
Hover over the file(s) you wish to remove and select.
Click the Remove button that looks like a trash can.
You can also select multiple files within the collection and select the ones you want to remove. Once selected a 'Remove' bar will appear at the top of the page.
Note: The file(s) will be removed from the collection but not deleted from the library.