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First you have to Override your Existing File so basically you need to follow such simple steps:

1. Create Module OSCP_NewsletterOveride - Path: app/code/OSCP/NewsletterOveride
2. Create module.xml file in path app/code/OSCP/NewsletterOveride/etc/module.xml

and paste this code

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="OSCP_NewsletterOveride" setup_version="1.0.0">
    </module>
</config>

3. Create etc/registration.php file -path: app/code/OSCP/NewsletterOveride/registration.php

and paste this code

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'OSCP_NewsletterOveride',
    __DIR__
);

4. Now Enable the Module with command:php bin/magento module:enable OSCP_NewsletterOveride

5. This way you can create your Module.

Now you need to Override file by Creating di.xml in etc folder. Set Preferences and Override File.