Custom Code in Head section

Hello,

The most easy way is to add it using a free plugin: https://wordpress.org/plugins/insert-headers-and-footers/

If you want to edit the header.php file directly then you need to copy the header.php file from the Parent theme to the Child theme and then try adding the code in it.

Regarding knowing how you can set up Child Theme, please check out the following guide: https://xtemos.com/docs-topic/setup-child-theme/

Further, regarding the main concern you can read the following Article for further help:
https://www.wpbeginner.com/plugins/how-to-add-header-and-footer-code-in-wordpress/

Hope this helps !

Best Regards