You will need to be able to use an FTP client or website control panel to upload files to and from your site.
The background at the top of the page in the header is an image that is located in the template files you are using. Example if you are using the default theme then the background image would be located in your store files at /assets/themes/default/background.jpg.
You can download this file and keep it as a backup for now. You can then modify that file, or create your own and upload it to the same location with the same name overwriting the original one. By default the image size is 699px X 200px. It is recommended to keep it that size.