I have a class in progress. The students have already completed one lab exercise. I need to add some new data files to their lab machines. Is there a way to do that without changing the lab template and republishing?

There are two options for accomplishing this task:

Add an additional Library

You can create an additional Library with additional files to add to the class.
Even if the class is in session, you can create a new Library, add files to it, then go into the Class to EDIT it and add the Library to the Class.  It will then be available to the Class Students. -- you will need to remind them to look for it and get various materials from it.  The files will be placed on the students' desktops under the "Class Files" folder.  There will be a subfolder under "Class Files" for each additional library added.  Some choose to use this method if they want the files organized for the student by exercise or chapter number.  Name your libraries with exercise number or chapter number in order to organize them under the "Class Files" folder in that manner.

Add files to the existing Library associated with the class

If the class already has a library associated with it, go to 'Libraries' in the left navbar.  Then, click on the library associated with your class.  Add your additional files to the library by dragging them into the drag/drop target.  Once the file(s) are uploaded, you'll see a recently added tag next to the additional items you added.  Now you can go back to you class already in session, expand the "Libraries" section at the bottom of the screen.  Click the green up-arrow icon under 'Actions' (over to the right) to push the items to the servers.  Instantly the students will be able see the files within their "Class Files" folder on their lab's desktop.

Also see:

Library Overview

Creating a Library

Deleting a Library