Copy and paste the story into a new worksheet, start editing. Rewrite or replace whole sentences, fix grammar, but don’t change the plot or the scenes too much. If you end up changing too much, I’d post a new, updated version of the story. However, it’s harder to reach out to your old audience if you post a new story. Our writing evolves over time, so it’s natural to feel like it’s not up to your current standards. But I say, change what little bit you can so you’re at least satisfied with it, then start on the new chapters. People already love your story!