You can add/remove files to/from a project with the Add/Remove Files dialog.
In the Project Editor
To add and remove files from a given project:
Make sure the Project Description File you want to add/remove files to/from is writable.
The File Types Filter
In the Project Tree, highlight the relevant project by clicking on it's name.
Choose Project > Add/Remove Files...
The Add/Remove Files dialog appears. At the top of the dialog, the currently selected
project and path are shown.
The File Types filter allows you to filter the view for individual, or all, file types included in the project.
Buttons in the Add/Remove Files dialog