The single post header is added via PHP as a Divi Library Layout.
Navigate to Divi > Divi Library in the WordPress menu.
In the library there is 1 layout under the category Archive Page Headers called Single Post Header.
Open the Post Title module to change how your post title is displayed. The background image is pulled from the featured images in your posts. You will also want to change the text in the module named Ophelia to your own.