At the moment, BlogniX only downloads your last 5 posts to a blog, which you can then edit. To view one of the last five posts, pick the post you want.
The post should show up in the area below. If this is the one you want to edit, click on the edit button and the text of the post should appear in the editing box above. Simply edit it, and then click on either post or postpublish. You just edited the post!