Where to put the function Wp_head in Customizr?

Hello
I have been using Customizr for a while along with the pluging headspace2. Small problem, I didn’t notice the following message:

“There are some issues with your theme that may prevent HeadSpace functioning correctly.
wp_head was not found in header.php”

Headspace might have never worked here with Customizr from what I understand.

I am using a child theme of customizr. In the backend editor I can access style and function, not the header.php

Where and how do I add this function please? Do I have to download the header.php through Filezilla, add it as listed then put it back? Is it the right place to put it?

My site if needed: https://www.laurentlanglais.com

Thank you!