When you install a theme and edit it in Wordpress, this tries to be clever and remember all your customizations so if you reinstall the theme you will have them all there. But sometimes when you fresh install a theme all you want is the original theme (which you have probably hosed with your carefree editing).

These are the steps to reset a theme to its original state.

1. First of all login as admin, and go to Appearance → Themes, here