How to override customerData folder's file in magento 2?

registration.php file,

filepath app/code/Package/Modulename/registration.php

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

module.xml file,

filepath app/code/Package/Modulename/etc/module.xml

<?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="Package_Modulename" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Checkout"/>
        </sequence>
    </module>
</config>

di.xml file, filepath app/code/Package/Modulename/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="Magento\Checkout\CustomerData\DefaultItem"
                 type="Package\Modulename\CustomerData\DefaultItem" />
</config>

DefaultItem.php file, file path, app/code/Package/Modulename/CustomerData/DefaultItem.php

<?php

namespace Package\Modulename\CustomerData;

class DefaultItem extends \Magento\Checkout\CustomerData\DefaultItem
{   
        //override function code here.....
}

you can change function code inside file function.

I have override one of file as below,

app/code/Namespace/Checkout/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">

 <type name="Magento\Checkout\CustomerData\Cart">
        <plugin name="checkout.cart.message" type="Namespace\Checkout\CustomerData\CartPlugin" sortOrder="1"/>
    </type>
</config>

And then I have added customer data file in below path,

app/code/Namespace/Checkout/CustomerData/CartPlugin.php

In which my class is as below,

class CartPlugin extends \Magento\Checkout\CustomerData\Cart implements SectionSourceInterface

and its method is as below,

 public function aftergetSectionData()
    {.. my code goes here}

Same thing you can do with defaultItem file as well.