Change Billing Details text on free checkout

billing-details-translation.php

<?php
add_filter( 'gettext', function( $translated_text, $untranslated_text, $domain ) {

	if ( function_exists( 'wc' ) && 'woocommerce' === $domain )  {
 
		//make the changes to the text
		switch( $untranslated_text ) {
		
			case 'Billing details':
				if ( ! WC()->cart->needs_shipping() && 0.0 === floatval( WC()->cart->get_total( 'edit' ) ) ) {
					$translated_text =  __( 'NOT billing details', 'interintellect' );
				}
			break;


		}
	}


	return $translated_text;

}, 10, 3 );