Customise header

Hi, I would like to customise the header. I am familiar with using child themes. I am aware that header.php calls a hook, i.e.

    /**
     * Hooked: onepress_site_header
     *
     * @see onepress_site_header
     */

But which file do I go to from here to edit and upload to my child theme? Your assistance is much appreciated
TIA
Bridie