You can only edit threads and comments for a short time after posting them.
You can open the editing page for up to 5 minutes after posting and make edits for up to 10 minutes after posting.
Make sure to review the content of your thread or comment before posting.
Editing a thread
Select "Edit thread" from the dot menu on the right side of the thread to open the editing page.
Editing a comment
Select "Edit comment" from the dot menu in the upper-right corner of the comment to open the editing page.