How to replace admin domain name with a new domain

[JTRGam3R]

Hello.
First sorry for my english.
I have an e-shop wordpress site with domain name exampleA.com on user2
and i have a domain exampleB.com without wordpress instalation(just empty) on user admin
and because the domain exampleB.com the person that had it before was sending spam email ,now the emails from my e-shop "exampleA" goes to spam folder on my costumers.So i bought an new domain "newexample.com" to resolve this issue with spam mails,
So the question is how to replace the problematic domain exampleB.com with the new i bought "newexample.com"
and how i replace and the name servers ? Also i use and cloudfare