Editing page attributes in Atahualpa

I’ve looked all over for a similar question but I can’t find one so please forgive any redundancies…

I’m using the Atahualpa template and would like to link to an outside site on one of my page attributes, but keep the permalinks for the other pages. I figured I could just go in and replace the page link with an href= link, but I’ve looked all over in vain for which .php file this code would be located.

Is there anyone using Atahualpa that can anyone point me in the right direction?

Thanks so much!