The current Google App Engine plugin for NetBeans does not have a UI interface to change the username and password that it has saved. It also does not display its properties in Preferences tab. The only way to change the stored username and password is to modify the deployment.properties file located in .netbeans/6.9/config/Preferences/org/netbeans/modules/j2ee/appengine
You simply need to change the email address and password in this file. Hopefully, the development will continue on this plugin to add this functionality in the future.