How to modify / extend a module in Magento 2 ?

In order to modify or extend any module structure and functionality, you should always create a custom module that extends the current one.

For example, if you wanted to extend or customize the Quick View extension, you would create a Quickview_Custom module, where you can apply the necessary code changes. The existing extension should remain un-customised in order to be able to upgrade to future versions without losing any of the changes you made. 

In the new custom module that extends the original, you can update XML templates, extend classes, etc. so as to match your requirements. 

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