Editing a file definition uses the same form as creating a file definition.
To access the edit page, open the File Definitions page and select the edit action on the right side of the file definition entry you wish to edit.
If you need information on what each file of a file definition is used for, consult the Add a File Definition page.
Please note that if a file definition is used by a task in a project, you cannot deselect that project while editing the file definition.