How to customize category template?

Here is your solution-

<?php
/**
 * The template for displaying category archive pages.
 *
 * @link https://codex.wordpress.org/Template_Hierarchy
 *
 * @package Codilight_Lite
 */
$cat = get_category( get_query_var( 'cat' ) );
$cat_id = $cat->cat_ID;
$child_categories=get_categories(
    array( 
        'parent' => $cat_id,
        // Uncomment the below line if you want empty category to appear on the list. 
        // 'hide_empty'   => 0
    )
);
get_header(); ?>
<div id="content" class="site-content container">
    <div class="content-inside">
    <div id="primary" class="content-area">
        <main id="main" class="site-main" role="main">
            <?php
            foreach ( $child_categories as $child ) { ?>
                <article>
                    <div class="category thumbnail">
                        <a href="" title=""><img alt="" src="" />
                        </a>
                    </div>
                    <div class="category-title">
                        <h2 class="entry-title"><?php echo $child ->cat_name;?></h2>
                        </header><!-- .entry-header -->
                        <div class="category description">
                            <?php echo $child ->description;?>
                        </div><!-- .entry-content -->
                    </div>
                </article>
            <?php
            }
            ?>
        </main><!-- #main -->
    </div><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>

Just copy and paste it to your archive.php and you are ready to go. Hope this is gonna help you.