@xiangyuman The support guide is here:
https://answers.netlify.com/t/support-guide-another-site-is-using-this-domain-verifying-domain-ownership-for-netlify-production-domain/13765
You can create a ticket with Netlify Support using the form here:
https://netlify.com/support
For ‘What kind of help do you need?’ you should select ‘Another site is using this domain’