Hi there, the credits are in the file footer.php.
The best way to make these sorts of changes to a theme is to use a child theme, so your tweaks won't be overwritten when updating the theme. Here are some guides in case you haven't made one before:
http://codex.wordpress.org/Child_Themes
http://op111.net/53/
http://vimeo.com/49770088
In your child theme, place a copy of footer.php.
Remove lines 16-19 and replace them with whatever footer text you like.
Just let me know if you need further help.