Maybe easier for you to find and employ a local gestor.
May save you time and money in the long run.

To change details, address etc. this has to be done at Traffico.
So around this area, it's Tarragona.
Once you have this done, you can pay car tax at the Base office, in our case Tortosa.

Just to add my 3c
Make the appointment online  - it will be for a few weeks in advance. Go for a morning one and get there early. As long as you have an appointment, they will see you. Do not forget to take a print out of the appointment with you - they will ask to see it.

Mine was for 10.00am, I went in at 9.30 and was out again in 20 minutes.
if you make it later - there are huge queues - I think the appointment times actually mean nothing, and the later in the day you are, the more of a back log there is.

HTH