Can't update a topic's body by API

@Lilly’s right.

A topic is defined by attributes like title, category, tags, etc.

But the content belongs to a post.

If you want to update the title and the content, you need 2 requests.

  1. PUT on the topic

  2. PUT on the first post. You can find the post’s ID in the HTML code of the page. Look for the data-post-id attribute in <article>.
    Example:

    <article id="post_2" aria-label="post #2 by @Lilly" role="region" data-post-id="1288311" data- 
    topic-id="264640" data-user-id="127856" class="boxed onscreen-post">