Are you a site admin or a teacher in the course where the forum is?
If you view the forum post, you should see some links in the bottom right of the forum post.
Clicking on ‘Delete’ will delete the post and any posts that stem from it. To keep the content of replies, try editing the post instead of deleting it.
Teachers and site administrators have the ability to delete and split a post in a forum.
Students may also delete their post, until someone else replies to it, or until the editing period concludes. Note: Most sites allow 30 minutes for changes an author might want to make.
There’s also a setting you can add to your config.php file which I always find very useful:
// Setting this to true will enable admins to edit any post at any time
$CFG->admineditalways = true;