in loop.php (if the child theme does not have this file, copy it from the parent theme) find (once near the top and similar near the end):
<?php /* Display navigation to next/previous pages when applicable */ ?>
<?php if ( $wp_query->max_num_pages > 1 ) : ?>
<div id="nav-above" class="navigation">
<div class="nav-previous"><?php next_posts_link( __( '<span class="meta-nav">←</span> Older posts', 'twentyten' ) ); ?></div>
<div class="nav-next"><?php previous_posts_link( __( 'Newer posts <span class="meta-nav">→</span>', 'twentyten' ) ); ?></div>
</div><!-- #nav-above -->
<?php endif; ?>
and remove/replace (either just the bottom one or both) with something like:
<?php /* Display navigation to next/previous pages when applicable */ ?>
<?php if ( $wp_query->max_num_pages > 1 ) : ?>
<div id="nav-below" class="navigation">
<?php if(function_exists('wp_pagenavi')) { wp_pagenavi(); } else { ?>
<div class="nav-previous"><?php next_posts_link( __( '<span class="meta-nav">←</span> Older posts', 'twentyten' ) ); ?></div>
<div class="nav-next"><?php previous_posts_link( __( 'Newer posts <span class="meta-nav">→</span>', 'twentyten' ) ); ?></div>
<?php } ?>
</div><!-- #nav-below -->
<?php endif; ?>
(the code is assuming you have the plugin wp-pagenavi http://wordpress.org/extend/plugins/wp-pagenavi/ installed and activated; and please note the different css ids for the top and bottom navs. )