My guess is if you want your friends to try out your update you could create a draft page and upload your game there. Then give your friends a secret url so they can play it on that page. That way you don't have to pull your game out of the competition to let them test it.

Then after the game jam is over you can move your update to the main page.

Yup, what Rythim Man wrote. Beside, updating the games during voting is locked, so if you'd remove the zip to upload a new one you wouldn't be able to and your entry's page would be left without anything playable.