Hi,
From your explanation, it's not clear what "child sub" is.
In Wordpress, themes ar located in a folder in the following path:
WORDPRESS/wp-content/themes (WORDPRESS is the location where you installed your Wordpress)
You will have to upload the theme you downloaded there and start editing the files inside it.
Most likely, you want to edit:
header.php
footer.php
main.php
single.php
page.php
...
Here are some links:
http://www.wpdesigner.com/2007/02/19...ss-themes-huh/
http://www.webdesignerwall.com/tutor...rdpress-theme/