Delete Generated Files in Final Cut Pro
Final Cut Pro allows users to delete render files and transcoded media associated with a Library, Event, Project or clip. It is sometimes necessary to force renders to regenerate. For example, if you have purchased a product after using the trial and you need Final Cut Pro to re-render your timeline in order to clear any watermarks.
The File menu is contextual depending on what you have selected. In the example we'll have the Project selected. To delete render files, follow these steps:
Select one or more Projects from the Browser
From the Menu Bar choose File > "Delete Generated Project Files..."
In the window that appears, select Delete Render Files and click OK
If you have selected to Delete Generated Library Files, you'll have the option to delete unused or all render files.
Refer to this article to manage transcoded media generated in Final Cut Pro.