If you have purchased your Domain from a different registrar. Registrars are companies such as 123 Reg, GoDaddy, Square Space, Wix.
Then you will need to log into your account on the registrar.
Name Server Records normally begin with
ns1.something
ns2.something
You will need to change these records to
ns1.hpdns.net
ns2.hpdns.net
Once you have changed these it can take up to 48 hours for the propagation of these records to take place.
The records are copied from your registrar to other servers around the world. You can check the progress of you change with one of the many available free tools such as this https://www.whatsmydns.net/
Please note: If you have been using a different service for your emails then by changing your Nameservers you will loose access to your email.
