How can I change nameservers

here (https://support.cloudflare.com/hc/en-us/articles/205195708-Changing-your-domain-nameservers-to-Cloudflare),
the video does not explain how to change the nameservers if the registrar is cloudflare
Actually, I have free trial acc, maybe that’s why I can’t find a tab where I can change nameservers.
Help me, please

There’s no need to change the servers, if your domain rigistrar is Cloudflare because you cannot register/transfer domains without having it active here.

Whats the domain?