Unable to locate header.php file in child theme

I cannot locate the header.php file in the child theme of my site. I need to access this as I must delete the Auto Adsense Code that for some reason has been left behind in the html doc (I already deactivated the insert header and footer plugin where the code was placed originally as well as deactivated the auto ads. The code includes <script async src=”//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js”></script>).

I have been to the appearance and theme but there is no editor option. I have also tried locating the file using sftp in filezilla and cannot locate the code in the header.php file. Am I doing something wrong?

Help would be greatly appreciated!

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