@gavindrake The footer copyright and “designed by” line are defined in functions.php around lines 560 – 580. I’d recommend creating a child theme for best results, but you could also change get_bloginfo( 'name' ) to your name for a quick fix.
However, this is a theme function so it will be refreshed when you update the theme. I’ve created a starter child theme that you can use if you wish to customize Rookie: https://github.com/ThemeBoy/Rookie-Child
With the child theme, you can create a copy of footer.php and make changes as needed 🙂