How does one change the grahic header

Each theme has an image folder, which is at wordpressroot//wp-content/themes/”yourtheme”/images, you can get the header image from here and modify it using any photo editing tool ..

Check my version of Kubrick header at
https://www.abhishekthakkar.com

Hope this helps..
-Thakkar

If you have Photoshop, you can use this template:
https://terral.111studios.com/?p=158

You might want to take a look at this too:
https://www.redalt.com/Tools/kubrickr.php