Swapping layout of category items with page items

I’ve been given an existing modified theme (Blogdrops) to work with and at the moment the categories run horizontally along the top of the blog underneath the logo image, while the pages are listed in the right sidebar. My client wants to swap these so the page items are listed along the top (as you would with a standard website) and the categories are listed in the sidebar. I’m still in the learning phase with WP and php; where would I look to modify this code and how?