Moving changed/hacked core files to Module

I have made changes to core files in dev environment to meet customer needs. Now I want to move all these hacks to a module so core can be updated properly. I am having trouble!

As a very simple example, I want to override the public function getContactLocationAttribute() in app>Models>Document>Document.php. It implodes an array with Zip before State but I need State before Zip with no comma between.
What is the correct way to achieve this? Thanks!