How to use ionic deploy with my existing app

I can see my app in my ionic.io account, but it’s Deploy tab reports: No Active Deploy. The app has the proper plugins in place. But if I try ionic upload, I get an error:

An error occured during your application - App ID XXXXXXX does not exist (CLI v1.7.16) .

The examples only discuss deploy from scratch. How can I use deploy with an existing app?

Thanks!
Z.