Here’s the website. It’s the same form as applying except you tick the box ( C ) to modify details.
http://www.aeat.es/mir/790mi/mod790_mi.pdf
It is a 3 sheet document, one for the bank , one for you and one for the office to keep. All three have to be stamped by the bank & I would assume that the fee is the same. The nie part doesn’t actually need updating but the residencia part does. Unfortunately they are now one form.