Removing Span tags in navigation

Question

I'm working on a custom Magento theme and I am stumped on how to remove the span class in the navigation.

I found the file which has it:

Mage => Page => Block => Html => Topmenu.php

I found the span which is in the <A> link but in the header of the file it says not to edit the file in case of a overwrite when updating.

Where would I place this file in my own theme's directory?

Thanks for any help.

Ben :)

P.S. I am using Magento ver. 1.9.0.1