The header of your Guestbook is a Press Article.  Access your Control Center and click on the Press Article Edit icon:

 

 

 

On the To Edit dropdown select Guestbook:

 

 

 

When the Guestbook press article loads, make any changes

you want in the Title, Subtitle and Memo1 fields:

 

 

If you want to add a picture, scroll to the botttom of the page and

use the Picture File Uploader.  When you're through click the

Save Changes button.