In order to delete/remove downloaded assets, please follow the steps below:
1- Tap 'Create', located on the bottom right of the screen.
2- Tap 'Create New' or a project thumbnail of an already created project.
3- Next to the video preview screen on the editing page, tap the Asset Store icon (store front icon).
4- Inside the Asset Store tap the 'My' folder icon on the top right.
5- Tap 'Remove' on the downloaded asset(s) to remove/delete.