How to upgrade existing project scaffolded with Yeoman

First make sure you have committed everything or have a backup, then just scaffold out over the project again. yo will ask you for each file if you want to overwrite. For those files you haven’t edited, just say yes. For the other ones, type d for diff and see what’s changed. Then manually do the changes, and run npm install & bower install to get the latest dependencies.

EDIT

As mentionned in comments by Markus Gattol :

yo <generator-name> now already run bower install & npm install i.e. no need to run them explicitly again – Markus Gattol