Wordpress archives of posts: Alphabetically order

functions.php

function custom_pre_get_posts($query) {
    // validate
    if(!is_admin() && $query->is_main_query()) {

        if(is_archive()) {
            $query->set('orderby', 'title'); // order posts by title
            $query->set('order', 'ASC'); // and in ascending order
        }
    }
}
add_action('pre_get_posts', 'custom_pre_get_posts');