function kraft_related_posts_headline( $headline ) {
$headline = sprintf(
				'<h3 class="jp-relatedposts-headline"><em>%s</em></h3>',
				esc_html( 'Other Things You Should Read' )
			);
return $headline;
}
add_filter( 'jetpack_relatedposts_filter_headline', 'kraft_related_posts_headline' );

Easy way to switch out the “Related” header that is in the Related Posts module in Jetpack 2.9+.