How do I upload an existing Tax Return document to the on-line system?

I already filed my return with the desktop application, but need to update the filing for  new 1099 that arrived late. Unfortunately this is a notoriously broken feature for Mac users and Intuit Support directed me to upload the return and make the modification. (They gave me a code to allow me to file the online version for free.) I'm logged into my account but don't see an option to bring the existing return into the system.