While in the calendar view, simply tap and hold on on a day to bring up trashcans on days with media. Then tap any of those trash cans to delete that video or photo. Just be careful not to delete anything you can't bring back.
You can also go into a calendar day and tap the icon on the top right (it looks like a film stripe) to reselect a second. This will bring up a screen with that days videos as well as the option to use Flux.
Both of these offer you a chance to delete and replace your snippet.