Updating a game during the voting process is considered unfair, because it would give you an advantage (more time than development) over the other participants.
If you want to update the game during this period, you will have to ask the Jam organizers, but it is unlikely that they will allow it.
The game is yours and you can really do whatever you want, like leave the Jam and upgrade if you want. But the competition is an event where many people participate and therefore the decisions that seek to make the competition as fair as possible for everyone must be respected.
Before participating in a Jam, it is best to make sure you read the rules and understand how it works.
If you want to leave, in the submission section of the Jam, you should have a button that says "Remove this submission" that allows you to remove your game from the Jam.