Hello,
1) You can find child theme archive in the Downloads area https://www.8theme.com/downloads or in full theme package that you downloaded from ThemeForest.
Download child theme and go to Appearance > Themes > Upload > install and activate child theme. Before child theme installation and activation go to Theme Options > Import/Export > export options. Once you activate child theme go to Theme Options > Import/Export > import options and reload the customizer page without saving changes!
2) What demo did you import? Very ofter users import demo that uses multiple headers feature but they edit only one header and forget to edit home page header. So, make sure that you don’t have multiple headers on your site https://youtu.be/BpeXfzNwkOc
Regards