If you're only a day or two into NaNo, you can usually safely scrap the start of your book and start over, at least if you think you can make up the word count. A week into it and you're hitting dodgy ground.
If you find that you really don't like the story you're writing then have the current characters meet the new characters, and the new characters take over and start a new adventure. Yeah, it's silly -- but there is no reason to drop all that writing, not in the case of NaNo. No, it won't be a coherent book or something you can sell in that format.
But if you are interested in publication later, you can hack off the first part, rework the opening, and make it into a better story.
But for NaNo -- just keep going!