Go to the parent theme directory and copy the file that you want to modify by child theme e.x header.php
Now go to the child theme directory and paste the header.php file following the parent theme directory path. For example, if you copy the content.php file from yourtheme/inc/ then you have to paste the content.php file into yourchildtheme/inc/
That’s it! Now you can modify the header.php file from the child theme.