When your domain is locked you'll see that the option to change your name servers is disabled in the Settings section of your Domain Control Panel.You'll need to unlock your domain before your name servers can be modified.

When your domain is unlocked, you'll be able to modify your name servers. Third parties might be able to initiate a registrar transfer for your domain if it is unlocked, so we recommend turning locking off only when you need to make changes to your domain name servers.

Note: Only the account owner can lock and unlock a domain.