You can edit the geese.html to change the geese movements or the fox.html file for the fox movements. The index.html file just allows you to tab between iframes of either page.

Reading the files may or may help you understand what is happening, but everything is there if you can figure out what it does. It may be easier or more educational to start from scratch.

Read this blog post about Fox and Geese CSS challenges or visit CSSHole for more CSS only projects.