Using plain java API, once you got a IChangeSet cs object,

  for (Object o: cs.changes()) {
   IChange change = (IChange) o;

 IVersionableHandle afterHandle = change.afterState();

IVersionable afterVersionable = wm.versionableManager().fetchCompleteState(afterHandle, new SysoutProgressMonitor());

IFileItem fileAfter = (IFileItem) afterVersionable;

string outputDir= "C:/temp";

        PrintWriter afterOnDisk = new PrintWriter(outputDir+"after", "UTF-8");
         String lineAfter;
         InputStream isAfter = contentManager.retrieveContentStream(fileAfter, fileAfter.getContent(), null);
         BufferedReader brAfter = new BufferedReader(new InputStreamReader(isAfter));
      while ((lineAfter = brAfter.readLine()) != null) {
       afterOnDisk.println(lineAfter);
         }
         afterOnDisk.close();

[.....]

}