To update to the latest Ionic 2 beta release:
npm install -g ionic@beta
If you already have project(s) started with an earlier release of Ionic 2 then you’ll have to update the project(s) as well (check the linked post for more details):
In such case, keep in mind that depending on how old release of Ionic 2 the project is using, you may hit a few (or many) breaking changes.