You can also email moderators at hn@ycombintor.com to request unflagging. I do that occasionally, with mixed results. (I've come to know which are long shots, and typically concede the point, but at least make the attempt.)
FAQ: <https://news.ycombinator.com/newsfaq.html>
Guidelines: <https://news.ycombinator.com/newsguidelines.html>
Both are linked at the bottom of most HN pages.
You can also email moderators at hn@ycombintor.com to request unflagging. I do that occasionally, with mixed results. (I've come to know which are long shots, and typically concede the point, but at least make the attempt.)