1/ Create a name server profile - In your account, go to item My Profiles > Nameservers profiles - Click on Add a new profile - Fill in the name servers then click on Save
2/ Apply a name server profile - Go to the item My domains > Manage my domains - Select the concerned domain - Click on Apply name server profile - Select the name server profile - Click on View profile then click on Apply profile
It will take up to 24-48 hours for your new name servers to be propagated worldwide. In order to avoid zone check errors and any website interruption, configure first your domain on the new name servers.