how to implement next/prev within category archive?

It sends you to the next/prev post by date of been published. I don’t know what gives you these next/prev links and where (single.php, category.php, archive.php or any other custom template file).

Without knowing more (some code example) all that I can give to you is this:

$page_nr="&paged=";
$page_nr = get_query_var( 'paged' ) ? $page_nr.get_query_var( 'paged' ) : ''; //<!-- tell wordpress this is paged
query_posts('cat=7&posts_per_page=6'.$page_nr); //<-- set cat= to the numeric category

if (have_posts()) {
    while (have_posts()) {
        the_post();

        // do your awesome WP loop stuff here
        <div><?php next_posts_link('Next Page &raquo;') ?></div>
        <div><?php previous_posts_link('&laquo; Previous Page') ?></div>
    }
}

That will help you, you may need to find how to get the current category that the user is currently browsing etc etc. Not the best fit, but it will help you to figure out the rest.