assuming you already work with a child theme, edit content.php in the child theme;
remove this section:
<?php if ( comments_open() ) : ?>
<div class="comments-link">
<?php comments_popup_link( '<span class="leave-reply">' . __( 'Leave a reply', 'twentytwelve' ) . '</span>', __( '1 Reply', 'twentytwelve' ), __( '% Replies', 'twentytwelve' ) ); ?>
</div><!-- .comments-link -->
<?php endif; // comments_open() ?>
depending on where exactly you want to move the ‘reply’ text to, you could add the removed section back into content.php, before or after this part:
<?php twentytwelve_entry_meta(); ?>
or edit functions.php of the child theme, and add a modified function twentytwelve_entry_meta()
into whihch you have integrated teh ‘reply’ link.