Hi there, after the frantic gamedev pace of the last 2 weeks, the panicky last-minute submission process (literally down to the wire), and a well-deserved breather for a good 12-hours or so, it's time to give the game - as is allowed - another look to make sure it's kinda playable. 

So I ask: how does the process work for uploading changes (always within the rules of course) to our games? Do we just upload a NEW zip file entirely, delete the old one? Is this straightforward to do on itch.io? 

Any insights or advice are welcome, thanks!

You can upload a new zip and delete the old one, yes. Or you can leave the old one and just upload the new one with the version number bumped up (if your release names have numbers in them).