Rename "Posts" to "Articles"

functions.php

<?php

function fwp_rename_post( $params, $class ) {
    if ( 'post_types' == $params['facet_name'] && 'post' == $params['facet_value'] ) {
        $params['facet_display_value'] = 'Articles';
    }
    return $params;
}
add_filter( 'facetwp_index_row', 'fwp_rename_post', 10, 2 );