If you'd like to work in another city, all you have to do is go 'online' in that city. The Fleet App will use location services to determine where you are and if Postmates is available in that area. If it doesn't switch right away, please try again a minute or two later, and make sure your location services are on.
Make sure your vehicle type is supported in your new market. You can review these options under Preferences on the Account page.