You have to remove action woocommmerce_email_header
remove_action( 'woocommerce_email_header', 'email_header');
Which was registered at
wp-content\plugins\woocommerce\includes\class-wc-emails.php
Than add your own action
add_action('woocommerce_email_header','your_function_name');
your function should output html
default there is below code in header template
<?php
/**
* Email Header
*
* This template can be overridden by copying it to yourtheme/woocommerce/emails/email-header.php.
*
* HOWEVER, on occasion WooCommerce will need to update template files and you
* (the theme developer) will need to copy the new files to your theme to
* maintain compatibility. We try to do this as little as possible, but it does
* happen. When this occurs the version of the template file will be bumped and
* the readme will list any important changes.
*
* @see https://docs.woocommerce.com/document/template-structure/
* @author WooThemes
* @package WooCommerce/Templates/Emails
* @version 2.4.0
*/
if ( ! defined( 'ABSPATH' ) ) {
exit; // Exit if accessed directly
}
?>
<!DOCTYPE html>
<html <?php language_attributes(); ?>>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=<?php bloginfo( 'charset' ); ?>" />
<title><?php echo get_bloginfo( 'name', 'display' ); ?></title>
</head>
<body <?php echo is_rtl() ? 'rightmargin' : 'leftmargin'; ?>="0" marginwidth="0" topmargin="0" marginheight="0" offset="0">
<div id="wrapper" dir="<?php echo is_rtl() ? 'rtl' : 'ltr'?>">
<table border="0" cellpadding="0" cellspacing="0" height="100%" width="100%">
<tr>
<td align="center" valign="top">
<div id="template_header_image">
<?php
if ( $img = get_option( 'woocommerce_email_header_image' ) ) {
echo '<p style="margin-top:0;"><img src="' . esc_url( $img ) . '" alt="' . get_bloginfo( 'name', 'display' ) . '" /></p>';
}
?>
</div>
<table border="0" cellpadding="0" cellspacing="0" width="600" id="template_container">
<tr>
<td align="center" valign="top">
<!-- Header -->
<table border="0" cellpadding="0" cellspacing="0" width="600" id="template_header">
<tr>
<td id="header_wrapper">
<h1><?php echo $email_heading; ?></h1>
</td>
</tr>
</table>
<!-- End Header -->
</td>
</tr>
<tr>
<td align="center" valign="top">
<!-- Body -->
<table border="0" cellpadding="0" cellspacing="0" width="600" id="template_body">
<tr>
<td valign="top" id="body_content">
<!-- Content -->
<table border="0" cellpadding="20" cellspacing="0" width="100%">
<tr>
<td valign="top">
<div id="body_content_inner">