Visit the website where you registered your domain name from. If it provides a control panel for domain administration, login into that and look for links where you can modify or change your DNS and add the following:
If your registrar was your previous web host and it does not provide you with a login to a domain administration panel then you must contact their support and ask them to change the Domain Name Servers (DNS) for you. Once the DNS have been changed, it can take up to 48 hours to propagate completely over the internet.
Domain Propagation is the period of time, or delay, involved in sending your domain’s address information to all the other name servers in the world.