It hasn't been submitted yet, just allow the old project, which I unintentionally submitted, because you don't allow people to see projects before the deadline and fix it?
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We just implemented this feature this game jam. You as the submitter should be able the edit the project since it makes it's own itch page. I believe tho, the edits should reflect the game subbed in the game jam. Submissions aren't locked so editing is allowed. I will test this out later as well to see what is happening.