In order to edit the build configuration of a multi-project session one must switch to the Project panel and perform the selection of the project and the configuration in the second half of the page.

The first half of this page is misleading as it shows the same selectors for the execution configuration (in an easier to read way to make things worse).

It would help if the execution configuration was moved to another place or if the two set of selectors were merged and only the editing of the current execution configuration was allowed.

I'm sorry for the lack of clarity of this report but I am running the French version of the tool and this page is only halfway translated.
Please request any additional details you might need.