If, and only if, you are the owner of a piece of media, then you can edit it.
Editing media means being able to change everything that makes it up (title, commentary and the media itself).
On the other hand, the media remains identified as such and will appear modified in experiences that already included it.
To delete it, you need to go back to the media in question and instead of editing it, click on the mini-menu to the right of the edit button and then on delete media.