In theory, updating your Angular application is simply a matter of updating the package.json Angular core library dependencies. However, depending on the features your legacy Angular 2 web application uses, there are a handful of changes in Angular 4 that must be addressed if you upgrade your application.
Following Angular Changes
The Angular team releases detailed change lists for every release of Angular on their GitHub repository. These changes have links to detailed documentation and explanations for the changes. It's recommended that you read through the change list before upgrading your Angular application to make sure you understand you are aware of any deprecations or major changes between your version and the version you are upgrading to.
https://github.com/angular/angular/blob/master/CHANGELOG.md
The Angular team releases detailed change lists for every release of Angular on their GitHub repository. These changes have links to detailed documentation and explanations for the changes. It's recommended that you read through the change list before upgrading your Angular application to make sure you understand you are aware of any deprecations or major changes between your version and the version you are upgrading to.
https://github.com/angular/angular/blob/master/CHANGELOG.md