How should I modify my code ?????
The original question was how to change the header image. I asked for a link to your site, and you said it's broken, so it wouldn't help. So you asked how to "generally" change the image.
I told you. Make your image. Rename it to whatever the current header image is. Upload it and overwrite the old image. That's the most general and quickest way to do what you want.
However if you need someone to tell you how to modify your code, we need to see a link. That's where all of the information we need is. Linking to the theme is fine (and for the record, the link you provided is incorrect), but we don't know what - if any - modifications you made that may have an effect on what you're trying to do.
I gave you a general, and very simple, answer. If you'd like me to provide code specific answers, then I need to see a link to the site in action.