Not sure if you found the answer yet. You can actually do this on the "watch.slingbox.com". Once you start watching there, click on "Options" and then "Slingbox Setup". You might get a pop-up asking you to "disconnect your video stream". Click "Continue". On the next screen, you will have 5 options like "Check for Updates", "Configure Inputs", "Set Up Program Guide" etc. "Set Up Program Guide" is where you will be able to configure a new zip code or choose the correct lineup.
Hope this helps.