Eleftheria11 Hi there, hope you're well. I must ask that in future - you raise a new thread for a problem, rather than commenting on an unrelated one. One of the main goals behind Ezoic Community is having a record of information that is easy for publishers to search through and find answers to their question. If a publisher has a similar issue to you, in the future, they're unlikely to find it in this thread.
That said, without knowing more specifics, updating the CNAME record for your subdomain is very unlikely to impact ad serving on that subdomain (and almost certainly won't impact ad serving on the rest of the site). This article will guide you through how to do so, but if you do run into any difficulties - please do feel free to raise a thread: https://support.ezoic.com/kb/article/how-to-update-add-dns-records-and-subdomains
-Lilla