Not currently, it is only editable within the PBI desktop client. Ability to change this in the server is on the roadmap to support in future versions. You *could* hack around this by calling the REST API to download the file and edit the file directly in script + republish.
You can change the connection string at the ReportServer DB level by updating the entity - DataModelDataSource , which has the connection string in the encrypted format.
It's simple get the conn. string from this table that you want to update and replace the existing one with the new one.