Go to Appearance > Themes > Install and activate the child theme.
Please note that the child theme does not inherit the theme options of the parent theme, if you enable the child theme, you need to import options from the parent theme.
Go to Theme Options (when the parent theme is activated) > Import/Export > download the export file. After that activate the child theme > go to Theme Options > Import/Export > import your options and reload the page without saving! Save options only after reloading. Check your site after that, and use the child theme.
