Just to clear some things up. The package called ionic on npm is for the Command Line Interface (CLI). So when you run npm install -g ionic@alpha you are updating the CLI which allows you to type commands such as ionic serve and ionic info.
The package called ionic-framework is what holds the source code for Ionic 2 and allows you to use Ionic components in your app. That can be found here: https://www.npmjs.com/package/ionic-framework
Normally when you want to add a package to a project you would install it with a --save flag and it would add itself to the package.json file. We have already added it though to the starters and conference app, so when you download the conference app and run npm install you are installing the ionic-framework package into node_modules in order to be used. The starter app runs npm install for you when you start a new project.
So for right now since the version of ionic-framework is already listed in package.json you can update it manually in that file to update to a specific version and then run npm install inside the project directory. You can also run npm install ionic-framework --save to update to the latest version. I imagine we will add a command to do this for you in the future but it hasn’t been discussed yet.
This is something we plan on going over in the docs in detail, but we haven’t yet. Let me know if anything I said was confusing. 