override adminhtml block magento

Go to custom module config.xml

this will overrite Mage/adminhtml/block/customer/group/edit/form.php
<config>
     <global>
              <blocks>
            <adminhtml>
                <rewrite><customer_group_edit_form>Bill_Checked_Block_Adminhtml_Customer_Group_Edit_Form</customer_group_edit_form>
                </rewrite>
            </adminhtml>
        </blocks>
     </global>
</config>





then create file in   app\code\local\Package\Module\Block\Adminhtml\Customer\Group\Edit\Form.php


use code


class Package_Module_Block_Adminhtml_Customer_Group_Edit_Form extends Mage_Adminhtml_Block_Customer_Group_Edit_Form
{
    protected function _prepareLayout()
    {
     ///Youre code is here
     }
}