Preference
Create the old school Magento 1 Rewrite, but it is not recommended.
Use the snippet in the Magento 2 module creator.
Files
etc/di.xml
<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<preference for="Test" type="Mage2Gen\Module\Rewrite\Test"/>
</config>
Rewrite/Test.php
<?php
declare(strict_types=1);
namespace Mage2Gen\Module\Rewrite;
class Test extends \Test
{
}
