Hi there,
go to Apperance > Elements and you will see a header element assigned to the Front Page.
Edit that to change the content, button link etc. The background image is the featured image of the page – so edit the home page and change the featured image.
More info here on header element:
https://docs.generatepress.com/article/header-element-overview/
Also if you preview the Site in our Site Library:
https://generatepress.com/site-library/
And visit the Blog there are a number of articles on how the site was built, which you may find useful