Header photograph help!

I am working on a website for a friend who initially had someone else build it for her. At the moment I have been editing the website for her, and she has asked me to change the photograph with the text that says “ecclestone valley miniatures” for her. I am quite new to this and am not sure where I can do this, is it under “appearance” and “editor”? the website is http://www.ecclestonevalley.com.au/

Any help at all will be much appreciated!