Sometimes when your uploading your app to Google App Engine if something goes wrong during the process, And when you retry to upload you get an error asking to rollback previous action. This is how to run that command.
C:\Program Files (x86)\Google\google_appengine> C:\Python27\python.exe appcfg.py rollback d:\wamp\www\projectname --oauth2
you can simplify this by adding python.exe and appcfg.py paths to your system path variables.
/usr/local/google_appengine/appcfg.py rollback [your-project-directory]