Access your Pagatelia on-line account at www.pagatelia.com. In the top menu select the option 'Mobes'. Click on the numbering (PAN number or VIAT number) of the device whose license plate you want to modify or add. Fill in the submenu fields and finally click on the "Save Changes" button.
You can modify it as many times as you need.
Remember that if you are traveling abroad (essential if you are traveling to Portugal or Italy) you must have the license plate number associated 24 hours before starting your trip.