None of the APIs for the more 'Word' like online editors seem to have any 'edit' functionality, just download and upload. This is obviously due to the complexity of the documents being stored.
With Google Docs API you can get a document, edit it, and then re-upload it (you may need to delete the previous version as well I think). Zoho seems to provide an 'update' method which combines the two operations.
Have you looked at the Google Notebook API instead? This might provide more granular access to the data in the 'note' due to its structured approach to storing the data.