Step 01:

Go to the parent theme directory and copy the file that you want to modify by child theme e.x header.php

Step 02:

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.