I want to change the name server(not DNS)

Usually, when I wanted to change the name server, I copied and pasted 4 addresses from the new name server site to the existing domain site. But I can’t seem to find a location in Cloudflare where I can change the name server. Does anyone know how to fix this?
I don’t want to choose how to change the DNS settings because I get something like “www” in front of the domain.

See here