Generally there are no straight forward instructions that can be followed step by step. However, you can use the following approach for changing images in almost any template for almost any script.

Right click on the image in your browser and select view image. Your browser will open an URL of type:

http://yourdomain.com/full/path/to/header.jpg

Where /full/path/to/header.jpg will be the actual path to the header image and its name.

You will see where exactly the image is located under your account.

Now you can now login via FTP client or cPanel -> File Manager and replace the header image with the custom header you have created.

Note that the full directory path when you access it via FTP or cPanel will include public_html in the beginning (the web root for your hosting account).

NB: Bear in mind that if the banner does not change after you have successfully made the modifications you will have to clear the cache for your browser and reload the page.