While logged into your Account Manager navigate to the Domain Name Registration section.
Click the Modify Name Servers icon.
From the Modify Name Servers page select the domain you wish to update under the Domain Names section using the checkboxes to the left of each.
Under the Change Name Servers section click on Custom Name Servers and then input the desired name servers in the fields below.
Once complete click the Update button in the lower right and you will be taken to the confirmation page.
Click the Confirm button to save the changes. Note that nameserver changes can take up to 24-48 hours for full propagation.