Customizing Header Image Height with Child theme in Twenty Thirteen

Hi there, I’m trying to increase the header image height to 400 px. I followed these instructions with my child theme:

https://www.ads-software.com/support/topic/how-to-change-twenty-thirteen-header-image-height

I created the child theme file in mytheme directory.
I added the CSS

And this is the error I got:

Fatal error: Cannot redeclare twentythirteen_setup() (previously declared in /home/content/57/5707057/html/wp-content/themes/mytheme/functions.php:81) in /home/content/57/5707057/html/wp-content/themes/twentythirteen/functions.php on line 108

Can anyone help?