you need to go to the "main" traffic dept. and go to the registration renewal section. from there they will ask you to pay 150QR for the replacement of your  plate number. once paid and registration renewed, you need to go to a separate building where they remove and install the new plates. submit a copy of your receipt and you'll be ask to comeback after 3 working days.

after 3 days go back and they will replace your plate with a new one :)