Quoting from the FAQ:
“Please don’t change the game on the itch.io page until after the voting period ends. If there’s a bug that seriously affects/prevents playability that’s a “quick fix”, please feel free to address it. Just leave a note on the itch.io page saying what you changed and why. Since the code is not being judged, you can update the GitHub repository any time.”
So, if your update fixes an important issue, or is a “quick fix”, then go ahead! Just remember to add a note on your game page ;)