How can I change the order of comments with wp_list_comments

This is what’s in the theme I’m using
<?php wp_list_comments('callback=custom_comments'); ?>

I need the comments to order from newest to old.

This is the custom_comments function

// custom comments
function custom_comments($comment, $args, $depth) {
	$GLOBALS['comment'] = $comment;
	global $commentcount;
	if(!$commentcount) {
		$commentcount = 0;
	}
?>

	<li class="<?php echo $oddcomment; if($comment->comment_type == 'pingback' || $comment->comment_type == 'trackback') {echo ' trackback';} else if($comment->comment_author_email == get_the_author_email()) {echo ' admincomment';} else {echo ' regularcomment';} ?>" id="comment-<?php comment_ID() ?>">

		<div class="authorinfo">
			<?php
				if($comment->comment_type != 'pingback' && $comment->comment_type != 'trackback') {
					// Support avatar for WordPress 2.5 or higher
					if (function_exists('get_avatar') && get_option('show_avatars')) {
						echo '<div class="authoravatar">';
						echo get_avatar($comment, 24);
						echo '</div>';
					// Support Gravatar for WordPress 2.3.3 or lower
					} else if (function_exists('gravatar')) {
						echo '<div class="authoravatar"><img class="avatar" src="';
						gravatar("G", 24);
						echo '" alt="avatar" /></div>';
					}
				}
			?>
			<div class="authortext">
				<?php if (get_comment_author_url()) : ?>
					<a class="authorname" id="commentauthor-<?php comment_ID() ?>" href="<?php comment_author_url() ?>">
				<?php else : ?>
					<span class="authorname" id="commentauthor-<?php comment_ID() ?>">
				<?php endif; ?>

				<?php comment_author() ?>

				<?php if(get_comment_author_url()) : ?>
					</a>
				<?php else : ?>
					</span>
				<?php endif; ?>
				<div class="date"><? printf( __('%1$s at %2$s', 'blocks2'), get_comment_time(__('F jS, Y', 'blocks2')), get_comment_time(__('H:i', 'blocks2')) ); ?></div>
			</div>
			<div class="count">
				<?php if($comment->comment_type != 'pingback' && $comment->comment_type != 'trackback') : ?>
					<a href="javascript:void(0);" onclick="MGJS_CMT.reply('commentauthor-<?php comment_ID() ?>', 'comment-<?php comment_ID() ?>', 'comment');"><?php _e('Reply', 'blocks2'); ?></a> |
					<a href="javascript:void(0);" onclick="MGJS_CMT.quote('commentauthor-<?php comment_ID() ?>', 'comment-<?php comment_ID() ?>', 'commentbody-<?php comment_ID() ?>', 'comment');"><?php _e('Quote', 'blocks2'); ?></a> |
				<?php endif; ?>
				<?php edit_comment_link(__('Edit', 'blocks2'), '', ' | '); ?>
				<a href="#comment-<?php comment_ID() ?>"><?php printf('#%1$s', ++$commentcount); ?></a>
			</div>
			<div class="fixed"></div>
		</div>

		<?php if($comment->comment_type != 'pingback' && $comment->comment_type != 'trackback') : ?>
			<div class="content">
				<?php if ($comment->comment_approved == '0') : ?>
					<p><small>Your comment is awaiting moderation.</small></p>
				<?php endif; ?>

				<div id="commentbody-<?php comment_ID() ?>">
					<?php comment_text() ?>
				</div>
			</div>
		<?php endif; ?>

		<div class="fixed"></div>
	</li>

<?php
	if ($oddcomment == 'oddcomment') {
		$oddcomment = '';
	} else {
		$oddcomment = 'oddcomment';
	}
}
?>

Any help is appreciated!