You'll need to edit the index.php, single.php and page.php files to do this.  Look for the <?php if ($flex_adsense_loc1 == "yes") code, and immediately after that you'll see a div.  The next 4 lines call the adsense, so if you want a different ad to display, simply replace those 4 lines (but not the closing div tag).  Do that in all 3 files, and also make sure you indicate "yes" to showing the location 1 leaderboard ad in the theme options.  That has to be set for that div to show.