<!-- <moduleDir>/etc/<area>/di.xml -->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<!-- ... -->
    <type name="Vendor\Namespace\Model\SomeClass">
        <arguments>
            <argument name="object" xsi:type="object">Vendor\Namespace\Model\SomeOtherClass</argument>
        </arguments>
    </type>
</config>

<!-- <moduleDir>/etc/<area>/di.xml -->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<!-- ... -->
    <preference
        for="Vendor\Namespace\Model\Example"
        type="Vendor\Namespace\Model\AnotherExample" />
<!-- ... -->
</config>

Above Example is a syntax of override core model.

Here is a list of points which will describe you how to make it possible

1. moduleDir - Extension directory Like app/code/custom/extension here extension is your directory in which all the necessary folders of extension will be placed.

2. area - area will be frontend or adminhtml

   • frontend - if extension will use functionality of frontend than di.xml will goes to in this folder

   • adminhtml - if extension will use functionality of adminpanel than di.xml will goes to in this folder

   • so it will be app/code/custom/extension/etc/frontend/di.xml or app/code/custom/extension/etc/adminhtml/di.xml

   • If wants to use both the functionality than di.xml file will goes direct in etc folder no need to put in frontend or adminhtml folder. Like - app/code/custom/extension/etc/di.xml

3. for="Vendor\Namespace\Model\Example" at here, the path of the file which will override functionality of the desired function.

4. type="Vendor\Namespace\Model\AnotherExample" at here, the path of the file which will provides functions which will override by step - 3

/**
 * @var \Vendor\Module\Helper\Data
 */
protected $customHelper;

/**
 * Constructor call
 * @param \Vendor\Module\Helper\Data $customHelper
 */
public function __construct(
    \Vendor\Module\Helper\Data $customHelper
)
{
    $this->customHelper = $customHelper;
    parent::__construct();
}