Below is a snippet that downloads (backups) an app from Google App Engine and stores it locally. I've been doing this before updating a previously uploaded application for security purposes.

appcfg.py download_app -e <your email>  -A <application name> -V <application version> <output directory>

If you don't want the previously entered account settings add "--no_cookies" and "-e <your email>" to the command string and you will be able to enter different credentials

appcfg.py download_app --no_cookies -e <your email> -A <application name> -V <application version> <output directory>