Sometimes you might want to change JRE or JDK associated with some Eclipse project. Right click your project and choose properties. Select Java Build Path -> Libraries, select JRE System Library, click Edit and choose whichever JRE or JDK you like. You can also add from this screen a new JRE or JDK.
If your project has facets and you just changed Java version, be sure to select Project Facets and change Java version. If you don't see Project Facets in this menu, don't be alarmed. Not all projects have them. As far as I know, it's mainly specific to Java EE.