header.php malicious code

You need to manually delete those code from a header.php file. you can check all core file like function.php, wp-config.php, index.php, footer.php any way you can found anused code script please remove it.

Your link above doesn’t seem to work. If you can access the header.php file from the theme editor (from the WordPress backend, go to Appearance > Editor), you can scroll down to see the code you need removed, remove it, and save changes. If you do not have access to the File Editor, you may need to access your theme files via FTP (root > wp-content > theme folder > header.php) and remove the code, upload the updated version to replace the current version.

You can also do a bit of scanning and review of any malicious code in your theme files with the Wordfence plugin, have a look at https://www.ads-software.com/plugins/wordfence/