From the Client Area, click Domains:
Then click the small arrow next to your domain (Step 1) and select "Manage Nameservers" (Step 2):
Select the "Use custom nameservers (enter below)" radio-box, and then enter your nameservers in provided fields and click the "Change Nameservers" button
Please note that nameserver changes may take up to 24 hours to fully propagate.
Note: If you have several domains, and you'd like to update nameservers on all of them, you can also use the "Bulk changes" option.
Please open a support ticket if you need any further assistance.