If you wish to delete some videos, just select desired video(s) from your Dashboard and click on the "Bin" icon at the top.
Sometimes can happen that leftovers appear under My Files. If there is no item under column "Used On", you can delete these files without any doubt, since they are not associated with any of your videos.
Finally, click on the button "Refresh Storage" and the amount of occupied storage will be re-calculated.
Our best recommendation for organizing your videos is to add them into groups (folders). You can upload multiple videos simultaneously via Mass Import option.