As we found out NIE certificates expire, the number doesn’t that stays with you for life.
You need to fill up an EX14 form take this along with your passport and old NIE certificate to a local main police station and you should be able to queue and get the new green certificate the same day (areas differ)
The first clerk will check your papers and then give you a D90 or D95 form, take this to the local bank and pay the 10 Euros, get the receipt and then return to the police station, you may be able to get the new certificate same day or you may be asked to come back another day and collect it.
Remember to take copies with you of all the documents.
You will then be the proud owner of the new green resident of Europe (Spanish Branch) certificate that will have the same number as your original NIE