Calling url service dynamically in WP8 project

I have a problem calling a web server in a WP8 project. In my project in VS, I added ServiceReferense (the webserver gives me the WSDL), VS created all the classes and their associated web calls, it also stores the login password for access. There is another web server with the same services and calls configured to a different url. Sometimes I need to switch the webserver im working with, but this requires changing the login and password, now when I try to call the second one, I get a 401 (unauthorized) error. How do I change the username and password in the automatically generated ServiceRefferences?

I am using VS 2013 and WP8.1 SDK. The project is based on Silverlight.