I don't think anybody has the ability to switch back anymore, but if you do it's under the options menu (upper right part of the screen) and there will be an option for Classic MSN View My understanding was when they started the roll out instead of just the beta, they were not going to let anybody switch back, so that option is probably not there.

Even if you could, they are just going to move you to the Live Version as they get rid of the old interface.