You can change your domain name nameservers (DNS) from your customer area. Below are the steps to follow:
1- Log in using your domain name username and password.
2- Click on "My domain".
3- Click on "DNS".
4- Choose "My own DNS", enter the nameservers provided by your web host and click on the "Validate" button.
After each change, you must wait a few hours for the propagation of the new parameters. For more information about DNS propagation, you can refer to this article: What is DNS propagation?