Código:

<nav id="nav-below">
  <h1 class="section-heading"><?php _e( 'Post navigation', THEMENAME ); ?></h1>
  <div class="nav-previous"><?php previous_post_link( '%link', '<span class="meta-nav">' . _x( '←', 'Previous post link', THEMENAME ) . '</span> %title' ); ?></div>
  <div class="nav-next"><?php next_post_link( '%link', '%title <span class="meta-nav">' . _x( '→', 'Next post link', THEMENAME ) . '</span>' ); ?></div>
</nav>