I think it is strange to redirect to login screen… Well, if the user is authenticated, maybe you can force a logout, then the platform will automatically redirect to login page. If that is the case, take a look at AuthenticationServiceBean.logout().
To change the URL based on the screen, you first need to make sure you are using Routing API in your screens. Then you can programatically open a screen and cuba will update the URL based on routing configuration.