To override controller, we need to create a new module in directory app/code/. 

Create directory app/code/Module_Namespace/Module_Name 

e.g. app/code/Letsknowit/Mymodule 

  1. Create file app/code/Letsknowit/Mymodule/registration.php 

 MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Letsknowit_Mymodule',
    __DIR__
);

2. Create a folder etc i.e. app/code/Letsknowit/Mymodule/etc 

   Under etc create file di.xml 

   app/code/Letsknowit/Mymodule/etc/di.xml 

    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    

3. Create View.php Controller file in Folder Letsknowit/Mymodule/Controller/Product 

 ie. app/code/Letsknowit/Mymodule/Controller/Product/View.php 

    /**
     *
     * @category    Letsknowit
     * @package     Letsknowit_Mymodule
     *
     */
    namespace LetsknowitMymoduleControllerProduct;
 
    class View extends MagentoCatalogControllerProductView
    {    
        public function execute()
        {
            // We can write our code here
            return parent::execute();
        }
    }