Tamisa says Oh of course the official way is to ask for the Hojas de Reclamación in the shop
Good advice but it should have been "Demand the Hojas de Reclamación in the shop" complete it and if not satisfied after 2 weeks, go to the Consumers office in the local town hall.
I suggest that when one has a complaint always complete the form, no matter what promises are made. If the shop etc put things right within the two weeks, great, throw away your copies, if they do not, you are on the right road.
The form consists of 3 sensitised pages. Along the inside (left) edge, written horizontally, is which form is for whom.
You take the customers copy and the consumer's office copy.
If the shop say they do not have the form, or will give it to you, call the LOCAL POLICE (policía local). They should attend, fine the shop on the spot and close it down until they have the form is available. It works, I have used in several times both for myself and for others.