It depends on where you are looking. You can remove the device from my.garmin.com by going to the myProducts tab, and you can add it back via the Register Now link on myProducts tab.
You can disassociate a device from a Garmin Connect account in Garmin Express by selecting the device from the dropdown on the left, clicking the Device link on the right, and then clicking the Delete Device from Express and choosing the appropriate option. Then you can add the device to Garmin Express by selecting Add New Device from the dropdown on the left.
