In some cases you will not be able to change the eclipse project facets through dialog boxes. It would happen many times when you want to downgrade your versions of facets used by your eclipse project. For example, If your current project configuration is Dynamic Web Module 3.0, if you try to downgrade to 2.5, it won’t allow. In my previous example I have explained how to convert Java project to Web Project using project facets. We can do it in the configuration file. Follow these steps and update the details.
1. Try Updating Using Project Facets Window
2. Open your project in the navigator view
2. Open org.eclipse.wst.common.project.facet.core.xml