How to override core files into app/code/Magento folder in magento2
I want to change core functionality so I need to override core files into app/code/Magento folder instead of directly changing that core files.
Core file:  vendor/magento/module-catalog-import-export/Model/Export/Product.php
override 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?