How to use ngUpgrade when upgrading Angular 1.6 -> 6

I'm trying to upgrade 1.6 angular project to angular 6. In the guide it says

Upgrading with ngUpgrade The ngUpgrade library in Angular is a very useful tool for upgrading anything but the smallest of applications. With it you can mix and match AngularJS and Angular components in the same application and have them interoperate seamlessly. That means you don't have to do the upgrade work all at once, since there's a natural coexistence between the two frameworks during the transition period.

How do I use ngUpgrade? The

How ngUpgrade Works

doesn't tell me that in a way that I'd understand. I installed Angular 6 and tried to search for 'ngUpgrade' but nothing showed up. Also tried to search from web without success.