go to backend catalog->product->selectyourproduct->design->custom layout update and write below code . 

    <reference  name="product.info">
       <action method="setTemplate">yourTemplatePath/yourTemplate.phtml</action>
    </reference>


or go to your theme local.xml or catalog.xml and write below code

     <PRODUCT_TYPE_grouped translate="label" module="catalog">  
         <reference  name="product.info">
             <action method="setTemplate">yourTemplatePath/yourTemplate.phtml</action>
         </reference>
     </PRODUCT_TYPE_grouped>