Magento : How to remove or rename ‘Add New’ button from Admin Grid?

For Rename the Add New Button from Admin Grid...

Go to YourNamespace -> YourModule -> Block -> Adminhtml -> YourFile.php
 $this->_addButtonLabel=Mage::helper('yourmodulename')->__('Add New Button Name');
For Removing the Add New Button from Admin Grid...
 
parent::__construct();
$this_removeButton('add');