This is one of those things which is going to be really easy if you've already found how to do it. If, like me, you hadn't found it, then it's not as clear as it might be. The obvious (to my mind) place to do it, the DNS tab of the domain record, doesn't let you do it.
Go to Hover, and from "your account" select "domains":
On the domain you want to change, select "edit", and then "edit nameservers":
Edit the nameserver values that are there - if you're doing this then I assume you have some idea what they are and what you want to change them to!
That's it, you're done. :-)