Just set the asset browser in delete mode to delete your uploaded assets.
Click on the library name to open up the asset browser from where you can manage your images, videos, and audio files.
Click on “delete mode”
Click on the assets you want to delete
Click on “Delete (#) assets”
Marked items will be deleted when you click on “Delete”.
You exit “Delete mode” by clicking “Cancel Delete mode.”
When deleting assets from your asset library, be aware that you will o longer be able to generate videos from storyboards that are using the deleted assets.