Hello,
You can find the header.php file inside wp-content > themes > Your theme file > header.php
Please note that if you make any kind of customization in the parent theme file, upon updating the parent theme, all your customization will revert. so, make all your customizations in the child theme so that the customization won’t be affected even after the update of the theme.
For any further query/confusion, feel free to reach back to us.
Best Regards!!