You just need to change the nameservers at your registrar’s like you’d change them in any other case as well. Just go to your registrar and change them there according to the steps your registrar requires for that.
A registrar is the company where you purchased your domain.
This is the account associated with the domain you purchased.
At the company where you purchased your domain.
To use Cloudflare as a CDN/proxy/DNS/security solution for your domain requires changing your nameservers to Cloudflare so that Cloudflare can provide DNS/proxy services for your records. That is done at your registrar.