For those that have domains with us you can do this in our billing system.
You can view all domains you have listed with your account.
Those that are purchased through us will show as a link and once clicked on will display all info on the domain.
In billing you can change any info for the domain such as DNS settings and Contact info.
If you use the Custom DNS settings then you will need to set each entry so as to route email and Web requests to the proper place.
If you are not sure how to set custom DNS then we recommend to just set the name servers to your account.