Adding a simple function to the child theme’s functions.php
<?php
// WooCommerce Checkout Fields Hook
add_filter( 'woocommerce_checkout_fields' , 'custom_wc_checkout_fields' );
// Change order comments placeholder and label, and set billing phone number to not required.
function custom_wc_checkout_fields( $fields ) {
$fields['order']['order_comments']['placeholder'] = 'Enter your placeholder text here.';
$fields['order']['order_comments']['label'] = 'Enter your label here.';
$fields['billing']['billing_phone']['required'] = false;
return $fields;
}
?>
Or you can use this great plugin WooCommerce Checkout Manager