The name servers of your domain can be changed at anytime from your control panel by navigating to [Domains] -> [Manage] (Pen-Symbol) on the right side of your domain. Below the box with the heading "Accessibility" you will find the link [Use custom nameserver].
Please note, that you have to specify all other name servers of your name server provider. As a minimum, at least two name servers have to be specified.
Please note, that some domain extensions (e.g., .de) require the external nameservers to announce your domain before you make the change at easyname. This means that you must already have set up the domain correctly at your name server provider, before the change can be made.