Magento - Change layout of category page

To change the page layout of category or category product listing page:

1. Go to your theme page.xml file - app/design/frontend/default/{your theme}/layout/
2. Search for <block type="page/html"
3. Change the layout in template="page/{your desired layout}"
4. Your desired layout may be 2columns-left.phtml or 2columns-right.phtml
5. Finally the code may looks like this        
   <block type="page/html" name="root" output="toHtml" template="page/2columns-left.phtml">