K.c you can do this by `Layout xml changes.` and you need change it `Layout 1column.phtml`.


    <action method="setTemplate"><template>page/1column.phtml</template></action>

See at  [http://www.amitbera.com/how-to-change-page-layout-in-magento/][1]

<CATEGORY_YOUCATID>
<reference name="root">
    <action method="setTemplate"><template>page/1column.phtml</template></action>
</reference>
<CATEGORY_YOUCATID>

Example:guess that your `category  id` is `10`.If you want to assign `1column` desing then you need 
to put below code local.xml at `app/design/frontent/ your package/your template/layout/`

 

   

    <CATEGORY_10>
        <reference name="root">
            <action method="setTemplate"><template>page/1column.phtml</template></action>
        </reference>
    <CATEGORY_10>

  [1]: http://www.amitbera.com/how-to-change-page-layout-in-magento/