The timeline or wall is there to represent an overview of site activity. When you post a photo, it goes to an album and a thumbnail also appears on the timeline. Ditto blogs, etc. If you were able to edit the timeline, you would also be editing the original post.
Provided you're the author, you should be able to go to the original post, be it a blog, photo or video and change the text or captions there. Those changes should then immediately appear on the timeline or outline. If it's a photo or video you wish to change, you need to delete it and upload it again. Once deleted, it will disappear from the timeline.
In other words, you should be able to change any text appearing in the wall, timeline or outline by editing the original article.