The Spanish authorities have published a detailed FAQ document explaining the new process for applying for residency or exchanging an existing document for a TIE:

http://prensa.empleo.gob.es/WebPrensa/salamultimedia/documento_enlaces/covid/GUIA_BREXIT_ENG.pdf

This document is also included on the UK Government living in Spain website which has been updated here:

https://www.gov.uk/guidance/residency-requirements-in-spain