Rewording "Older posts" / "Newer posts"

create a functions.php?

yes;
http://codex.ww.wp.xz.cn/Child_Themes#Using_functions.php

in there, add the function with the edited code (replace the ‘Older posts’ etc):

function twentythirteen_paging_nav() {
	global $wp_query;

	// Don't print empty markup if there's only one page.
	if ( $wp_query->max_num_pages < 2 )
		return;
	?>
	<nav class="navigation paging-navigation" role="navigation">
		<h1 class="screen-reader-text"><?php _e( 'Posts navigation', 'twentythirteen' ); ?></h1>
		<div class="nav-links">

			<?php if ( get_next_posts_link() ) : ?>
			<div class="nav-previous"><?php next_posts_link( __( '<span class="meta-nav">&larr;</span> Older posts', 'twentythirteen' ) ); ?></div>
			<?php endif; ?>

			<?php if ( get_previous_posts_link() ) : ?>
			<div class="nav-next"><?php previous_posts_link( __( 'Newer posts <span class="meta-nav">&rarr;</span>', 'twentythirteen' ) ); ?></div>
			<?php endif; ?>

		</div><!-- .nav-links -->
	</nav><!-- .navigation -->
	<?php
}