1. Login to your account.
2. Click on Domains, then My Domain.
3. Select the domain you would like to work on.
4. From the list of options on the left side of the page, click on Nameservers.
5. Check Use custom nameservers (enter below)
6. Change the nameservers.
Click on the Change Nameservers button after editing the nameservers.
Please be note that changes can take up to 24 hours to propagate.