Determine the target Project
Data sources get published to DataSelf Web Server in a Project. (Out of the Box, DataSelf publishes the data sources to the "Data Sources" project.)
To modify an existing data source, you must know which Project it is currently in. Otherwise, if you publish the modified data source to a different project, it will be considered a separate data source and the first version's reports will not be affected. What a confusing mess!
To locate the existing locations, on the Web Server, the Content menu > Data Sources tab, see the Project column.
For simplicity, DataSelf recommends keeping all the data sources in their own "Data Sources" project and setting this project's permissions to "Managed by the owner...".