Hi! I was wondering if, in a few days time, I uploaded a "beta" version of my game (just in case  I get no further progress), could I upload a better version at a later point in time, before the jam finishes? I want to upload a completed game, but reupload a new version after I've added some polish and other quality of life improvements

thanks

As per the way Itch.io works: yes. 

Just submit your game to the jam as is, and you have all the way up until the end date to upload new versions of your game. 

What I do is upload new packages and hide the old ones so as to keep the download / play stats