Hey - Thanks for asking! You can change the public-facing version by clicking "Edit project" on the page. This will only affect changes that are publicly visible. We have a snapshot of the submission at the deadline which judges see.
Go ahead and make changes, but please do not delete your old video because it won't show up for the judges.
Oh I see. No. The rules say that during the certification period, "Makers will be allowed to make minor changes (such as bug fixes) during the Certification Period to move through the certification phase. No substantial changes will be allowed, and all Skills that do not pass certification by the end of the Certification Period will be ineligible for prizes in the Competition."
You can only make small changes required to pass certification during this period. That said, after the challenge we'd love to see you keep expanding on it!