How to modify / extend a module in Magento 2 ?

In order to modify / extend any module structure and functionality you should to create another custom module that extends the current one.

For example f you want to modify the Quickview module, create another 'Quickview_Custom' module, where you can make the necessary code changes. You should keep the existing module un-customised in order to be able to upgrade to future versions without loosing your changes. 

In the new custom module, that extends default module, you can change XML templates, extend classes, etc.. in order to match your requirements. 

More here: How to customize css / xml / phtml / js files in Magento 2 - examples [ Tutorial ]