You can delete files and folders by tapping the menu button then "My files". Navigate to the directory where the item you want to delete is located then follow these steps. You can only delete files and folders you backed up from your current phone.
Deleting a single file or folder
- Tap the three vertical dots.
- Select "Delete"
Deleting multiple files at once
- Tap and hold the first file you want to delete
- Single tap the other files
- Tap the Trash icon at the upper-right portion of the screen