Change a field label on checkout page

I want to change the wording “Additonal Information” and the wording “Order Notes” on the WooCommerce checkout screen to something else. (But I’d settle for just changing “Order Notes”.) This is all I want to change. In searching on this forum I found the following code:

//Change the Billing Details checkout label to Your Details
function wc_billing_field_strings( $translated_text, $text, $domain ) {
switch ( $translated_text ) {
case 'Billing Details' :
$translated_text = __( 'Your Details', 'woocommerce' );
break;
}
return $translated_text;
}
add_filter( 'gettext', 'wc_billing_field_strings', 20, 3 );

Now this is for changing “Billing Details”. I don’t want to change that but I tried it anyway so if that worked I could change it to what I want. I pasted it to the end of the file functions.php. But it didn’t do anything. I pasted it at the beginning. No joy. What am I doing wrong? How can I debug it?