You go the TomTom website, and change the device on your account (You have to use the same account that you purchased Traffic services with).
First, log on to your account. Then go to Plus Services. You should see a "select your device" Icon on the top of the right side of the screen. After you select your unit, you should see a place to enter your new device code on the bottom, Do so and save (if you had changed it in the last 6 months, you would have to get TT support to do it for you).
Then on the left side, you should see your traffic subscription underlined. Click on it, and it will give the option to "reset account". Do so, and if you are lucky, when you next try the receiver, you should be back in business.
If not - call TomTom support and they can manually set it up for you - they do it all the time.