This application does not exist (app_id=xxx)
this way worked:

modified the app.yaml to specify the application: some_id
python appcfg.py update ../some_application/server/python
I was unable to upload to an AppEngine as appcfg was telling me :

This application does not exist (app_id=u'xxx').

I was only a developer on the AppEngine, so as I was just testing I created a new AppEngine where I was the owner but I still get the same message on a newly created AppEngine.