A Debian or Ubuntu system can be upgraded remotely from a ssh session, even if the upgrade includes replacing the ssh deamon and the kernel (which requires to reboot the system). A upgrade via NX would be technically similar, however since NX is not part of Ubuntu, this cannot possibly be officially supported.
Either find out, how to perform the update via the command line and ssh or try the GUI and be prepared to recover via ssh if something goes wrong.
Let's see, what could go wrong:
new kernel does not boot on your hardware
Network could not fail for one of the following reasons
- after kernel update: driver module fails
- network-manager doesn't detect your network or expects user interaction (I'm not sure whether using network-manager is a good idea on server-like machines.)
- incomplete upgrade (after failing to recover from a unrelated problem) leaves network-manager or kernel and modules in a broken state
NX does not work after upgrade/reboot
The typical Ubuntu upgrade process does not disconnect you from the network unless for rebooting. And network should come up automatically after upgrade/reboot.
Personally, I'd give it a try if the machine is not mission-critical.