I did follow the docs  and, however, it only update the topic’s title, without updating the body.
My code (Javascript):
async function updateDiscourseTopic(topic_id) {
  const Discourse_API_Key = "Discourse_API_Key";
  const url = `https://discourse.mysite.com/t/-/${topic_id}.json`;
  const payload = {
    title: 'session_1',
    raw: "new body(doesn't apply)"
  }
  const response = await fetch(url, {
    method: 'PUT',
    headers: {
      "Api-Key": Discourse_API_Key,
      "Api-Username": "system",
      "Content-Type": "application/json",
    },
    body: JSON.stringify(payload)
  });
}
 
            
              
            
           
          
            
              
                Lilly  
              
                  
                    May 11, 2023,  1:01pm
                   
                  2 
               
             
            
              just a guess from looking at the code, but perhaps it may be because you are using topic_id instead of post_id? because first post is the topic in Discourse, more or less?
             
            
              3 Likes 
            
            
           
          
            
              
                Canapin  
              
                  
                    May 11, 2023,  3:31pm
                   
                  3 
               
             
            
              @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.
PUT on the topic
 
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>.
<article id="post_2" aria-label="post #2 by @Lillinator" role="region" data-post-id="1288311" data- 
topic-id="264640" data-user-id="127856" class="boxed onscreen-post">
 
 
             
            
              3 Likes 
            
            
           
          
            
            
              Wow,you are right, it works  
And I figured out how to retrieve the ID of the first post in a topic using the Discourse API:
async function getFirstPostIdFromOnetopic(topic_id) {
  const Discourse_API_Key = "Discourse_API_Key";
  const url = `https://discourse.mysite.com/t/${topic_id}/posts.json`;
  const response = await fetch(url, {
    method: 'GET',
    headers: {
      "Api-Key": Discourse_API_Key,
      "Api-Username": "system",
      "Content-Type": "application/json",
    }
  });
  const data = await response.json();
  const firstPostId = data.post_stream.posts[0].id;
  
  return firstPostId;
}
Thank you and @Lilly  
             
            
              3 Likes 
            
            
           
          
            
              
                system  
              
                  
                    June 10, 2023,  4:12pm
                   
                  5 
               
             
            
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