How to download metal 2?

First, let me clarify that Metal 2 is really and extension of Metal and so if you code for Metal you simple get Metal 2 by using some of the newer features added to the Metal 2 API.

Metal is a framework (i.e. a bundle of libraries and headers) provided by the OS. So you don't need to download Metal. But if you want to develop an app using Metal, you'll need to get Xcode and the SDK for the OS that. you're looking to develop for. (This is true if you're developing any app, not just a Metal app). Metal is just a piece of that SDK so there is nothing you need to download beyond that.