How do I override core files in app/code/Magento folder in Magento 2?

I want to change core functionality so I need to override core files in the app/code/Magento folder instead of directly changing that core files.

Core file:

vendor/magento/module-catalog-import-export/Model/Export/Product.php

Overriden at:

app/code/Magento/CatalogImportExport/Model/Export/Product.php

But it is not working. I don't know how to implement this task. Can you please help me?