How can I change author permalink structure from author's nicename to…

I know how to set up a child theme. But I’m barely literate in php.

I want to change the author permalink structure from:
domain.com/author/author-nicename
to
domain.com/author/lastname-firstname

I know that author-{nicename}.php is used somehow to create the structure… but I have no idea how to make the change in a child theme. And I don’t know the code for the lastname or firstname.

Can someone point me to what PHP file I would add to my child theme and what code to put in it?

Sincere thanks! ?