There are two ways to applying tax class to all products.

1/ The simple way is using update attribute product function of magento

Log in to your admin panel, then select : Catalog->Manage Products

Then you can click Select All, and from the Actions drop down on the right, choose Update Attributes. Here you can choose the new tax class



2/ The other way is using code to apply new tax class for all products:
Default value of tax class None is  0.

You can navigate drop-down tax classes in product section to find the value of tax class

Create the test.php in your root directory with the content as below. Then excuting this file via url : yourwebsite/test.php

header('Content-Type: text/html; charset=utf-8');

$productIds = Mage::getResourceModel('catalog/product_collection')->getAllIds();

array('tax_class_id' => 0),



Happy coding !