First of all, MAKE SURE NOT to register the new unit otherwise you cannot use the serial number to replace the old one in the old account!!!!!
and log in your old account.
Once logged in, at the upper part of the screen click on:
Then once in the MY PROFILE page shows up, replace the old serial number by the new one to update.
(MAKE SURE you enter the NEW serial number CORRECTLY, it is 12 digit long!!!)
Click the button just below the serial number area to change it and confirm on the next pop up.