As a security measure it is advisable to hide the server name.
I don't know who to set it up by hand. I searched through Jester's settings document (and code) but couldn't find one that sets the server name. As an example, Beego (the framework I have used in Go) allows setting the servername using config: https://beego.me/docs/mvc/controller/config.md
If you use httpbeast, its part of the serverInfo constant defined here:
https://github.com/dom96/httpbeast/blob/master/src/httpbeast.nim#L50
and used here:
https://github.com/dom96/httpbeast/blob/master/src/httpbeast.nim#L333
You could always fork httpbeast and replace that line.
"As a security measure it is advisable to hide the server name." - Never head about that. I can see how it can help in theory, but ... feels like grasping at straws.
Though in practice, attackers don't really check the Server: header. They just try every security exploit they know of, whether your server gives any indication of being vulnerable or not. Removing the Server: header is a security by obscurity action, and an almost entirely ineffective one. But if it makes you feel better, or you're being told to do it by your boss or an auditor, go for it. Just don't expect it to result in any significant improvement to your security posture.