The introduction of the Report Editor has meant that most of the XML code editing that used to be required to create custom reports can now be achieved through the Report Editor user interface. However, there are two scenarios that did not make it into this first version of the report editor.
To edit reports you will need a code editor.
If you have the correct server access, you can use SharePoint Designer to navigate to the BrightWork Reporter library and edit the report XML files directly. This is the recommended method for editing Query Sources.
If you do not have the correct server access, you can still use SharePoint Designer (or another code editor) to edit the reports. Download the report to your desktop and make your edits there. When you are finished, upload the report back to the BrightWork Reporter Library and ensure Overwrite existing files is selected.
If you have Visual Studio or Visual Studio Community, you can take advantage of Visual Studio's Intellisense for code validation and prompting.