How do I get to the functions.php

Okay. So I’ve been receiving the error Catchable fatal error: Argument 1 passed to Genesis_Admin_CPT_Archive_Settings::__construct() must be an instance of stdClass, instance of WP_Post_Type given, called in /home2/tiamarie/public_html/wp-content/themes/genesis/lib/admin/menu.php on line 122 and defined in /home2/tiamarie/public_html/wp-content/themes/genesis/lib/admin/cpt-archive-settings.php on line 38

I’ve been told to go to the functions.php. OKAY. But how do I go there? Is it the files that have been downloaded to my computer? If that’s the case, then which one because I have four functions.php files in my computer. Do I put the following code that was suggested to me at the end of all four files?
/**
* Disable Genesis CPT Archives
*
* This is useful to temporarily disable the Genesis CPT Archives,
* if you can not log into your WordPress site
* after updating to WordPress 4.6.
*
* See
* https://salcode.com/wp46/
* for more information.
*/
add_filter( ‘genesis_cpt_archives_args’, ‘fe_modify_args_to_include_non_existant_name’ );
function fe_modify_args_to_include_non_existant_name( $args ) {
$args[‘name’] = ‘this-is-a-name-that-does-not-exist-resulting-in-no-matching-cpts’;
return $args;
}

Is there a YouTube video anyone can suggest that demonstrates what to do?

Thank you.