You can edit a post that you have already added by clicking on the "Edit" button that appears on the post.
The Edit function is set up to discourage people from editing their post for the wrong reasons, so you can only edit it within 5 minutes of posting - leaving you a bit of time to correct spelling mistakes and so on.
Note that you can only save changes if you describe the changes you have made in the Edit Notes box. Again, this is to discourage abuse.