1) For that, You need to create app/design/adminhtml/Vendor/Theme/Magento_Backend/layout/admin_login.xml and paste the below code:
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="admin-login"
xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="logo">
<arguments>
<argument name="logo_image_src" xsi:type="string">images/admin-custom-logo.png</argument>
</arguments>
</referenceBlock>
</body>
</page>
2) Now, Make sure admin-custom-logo.png file available on app/design/adminhtml/Vendor/Theme/web/images/ this path.