How to change meta data(title, description) for current post?

Try this:

New title

add_filter('wp_title', 'filter_posttitle');
function filter_posttitle($title) {
 $title="New title ".$title;
 return $title;
}

and new content add code in header.php

<?php if ( is_single() ) { ?>
    <meta name="description" content="" />
<?php } ?>