1) Login to your WebHostingZone account at https://www.webhostingzone.org/members/clientarea.php
2) In your account go to "Domains" >> "My Domains"

3) Click "Manage Domain" next to domain you would like to edit
4) Go to "Nameservers" tab, check "Use custom nameservers" box and put your nameservers there. Click "Change Nameservers" button.

Please note: due to DNS propagation delay it might take from 24 to 72 hours for your changes to take effect.