We have to add the bellow codes to function.php file of active theme.
add_filter( 'woocommerce_cart_shipping_method_full_label', 'change_cart_shipping_method_full_label', 10, 2 );
function change_cart_shipping_method_full_label( $label, $method ) {
$has_cost = 0 < $method->cost;
$hide_cost = ! $has_cost && in_array( $method->get_method_id(), array( 'free_shipping', 'local_pickup' ), true );
if ( ! $has_cost && ! $hide_cost ) {
$label = $method->get_label() . ': Free';
}
return $label;
}