The post can be edited using the tooltip icon. Select "Edit the post" in the menu displayed and confirm the edit by clicking "Save". You can cancel a post as long as there are no responses to it. To cancel a post, select "Delete post". The owner of a thread in a non-course discussion forum can cancel (entire sub-threads) at any time, in a course-related discussion forum only the teacher can do this.
It's only appropriate to edit the meaning of a post in a more fundamental way shortly after it has been posted, otherwise you confuse readers who have already read it (it is marked as read for them). Later on, you should rather make a new post.
If you want to cancel an entire discussion forum, you can only do so for the Topic-based Discussion Forum. To cancel a discussion forum, click on the tooltip icon at the top of the row next to the discussion forum name and select "Discussion settings", then at the bottom of the form there is an option to "delete this forum". To avoid confusing readers, it is not advisable to delete a forum if they have already contributed to it. Only the discussion forum owner has the right to cancel the discussion forum.