How to modify a title tag in genesis?

Try this to create your custom titles.

remove_filter( 'wp_title', 'genesis_default_title', 10, 3 ); //Default title
remove_action( 'genesis_site_title', 'genesis_seo_site_title' ); //Genesis Extra Stuff
remove_action( 'genesis_site_description', 'genesis_seo_site_description' ); //Genesis Extra Stuff

add_filter( 'wp_title', 'genesis_default_title_new', 10, 3 );
function genesis_default_title_new( $title) {

    $title="Hello World!";
    return $title;
}