If the domain that you wish to make the DNS changes for has been purchased through us, you can manage this through the 'Client Area'. Please read the following help article on how to do this - What are your name servers?.
If you did not purchase the domain from us, we will not be able to change the DNS for you because the domain name is regrettably not in our control. Most commonly the place where you purchased the domain from (the registrar) will provide you with an account login which you can access to change the DNS records. Once you have access to this, you can then change the DNS to our DNS servers.