Change Single Page Navigation Text and Styling

In single.php replace:

// Previous/next post navigation.
twentyfourteen_post_nav();

 with this:

<nav class="navigation post-navigation" role="navigation">
   <h1 class="screen-reader-text"><?php _e( 'Post navigation', 'twentyfourteen' ); ?></h1>
   <div class="nav-links">
   <?php
     if ( is_attachment() ) :
      	previous_post_link( '%link', __( '<span class="meta-nav">Published In</span>%title', 'twentyfourteen' ) );
     else :
       previous_post_link( '%link', __( '<span class="meta-nav">&lt;&lt;Previous</span>%title', 'twentyfourteen' ) );
       next_post_link( '%link', __( '<span class="meta-nav">Next&gt;&gt;</span>%title', 'twentyfourteen' ) );
     endif;
 ?>
  </div><!-- .nav-links -->
</nav><!-- .navigation -->

 Change text and html as needed