How to change the " tab title " to a subpage ?

didn’t get it , the tab title is the meta-title ?
where can I change it ?
sorry , I’m new to all this stuff.

<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title><?php echo is_home() ? bloginfo('name') : wp_title(""); ?></title>
	<?php endif;?>

What should I change exactly ? thanks.

It isn’t , here’s the website : here

Here’s the theme : here