1. Select My Account.
2. Login to your account.
   
   
   
   
3. Select Domains.
   
   
   
   
4. On the domain you need to modify the NS press on Active.
   
   
   
   
5. Select Nameservers from the sidebar then choose Use custom nameservers and modify the NS then press Change Nameservers.
   
   

Note: Changing the NS may takes 24H till be propagated globally.