header title

My site :www.smartwayvoyager.com, My theme : Lifestyle child
Here are the instuctions :
Under Presentation -> Theme Editor in the WordPress admin, select “Header” from the list and replace <title><?php bloginfo(‘name’); wp_title(); ?></title> (or whatever you have in your <title> container) with <title><?php if (function_exists(‘seo_title_tag’)) { seo_title_tag(); } else { bloginfo(‘name’); wp_title();} ?></title>

I am not sure how to do it, I have got
Header under stylesheet style.css

[CSS code moderated per forum rules]

Where doI change it ??
and the instuctions mention wp-title which I do not have
Can someone help?