So you've just spent 10 minutes writing a very elaborate post, when you realize you spelled the name of the person you're replying to wrong.  How embarrassing!  Well...not really. 

Simply go to the bottom right corner of your post and click "edit," make your changes, and submit it again.  If all else fails, you can also delete your post down in that same bottom right corner.  The system will check to make sure that you really want to delete your post - just push "OK" and you're free from embarrassment for another day.