Changes to domain name information in registry-controlled DNS zones are generally not instantaneous.
Once you make changes to your domain with the registrar you purchased your domain from, the changes will first have to be uploaded to the registry, which will then update its DNS servers with the new information. ISPs DNS resolvers can then pick up this information through a sequence of stages. [...]
As a result, it may take several days for changes to a domain's DNS records to propagate throughout the Internet; firstly the information must propagate from your registrar to the registry, secondly the registry must update its own DNS servers, and thirdly the ISPs recursive resolvers must forget their cached copies of any previous settings.