If you have purchased a domain from us, kindly login to your APC billing account at https://billing.apc.sg/clientarea.php and follow the below guide to do so.
To elaborate if you have a dedicated IP from APC (for shared hosting/VPS/Dedicated Server) and wish to register a name server, the function you should use is the "Register a NameServer Name" and should use the below information to update both of your nameservers.
Nameserver : ns1
IP Address : 1.2.3.4 (Sample)
Nameserver : ns2
IP Address : 1.2.3.4 (Sample)
However if you wish to modify your name server, the function you should use is the "Modify a NameServer IP" and should use the below information to update both of your nameservers.
Nameserver : ns1
Current IP Address : 9.8.7.61 (Sample)
New IP Address : 1.2.3.41 (Sample)
Nameserver : ns2
Current IP Address : 9.8.7.62 (Sample)
New IP Address: 1.2.3.42 (Sample)
