I’ve just upgraded NXServer Free Edition on Ubuntu Hardy Heron see below for instructions:

Download the application from the NXServer  website from the link above and install using the following commands.

sudo /etc/init.d/nxserver stop
sudo dpkg -i nxclient_3.3.0-6_i386.deb
sudo dpkg -i nxnode_3.3.0-12_i386.deb
sudo dpkg -i nxserver_3.3.0-15_i386.deb

After installing using the commands above, you need to do the next step otherwise you’ll get the following error message:

NX 500 ERROR: NX Server public key '/usr/NX/home/nx/.ssh/default.id_dsa.pub' doesn't exist.

Try these command to solve the problem:

sudo -i
mv /usr/NX/home/nx/.ssh/default.id_dsa.pub.backup /usr/NX/home/nx/.ssh/default.id_dsa.pub
sudo /etc/init.d/nxserver start
exit

Alternatively, you could just move the file first, upgrade, then copy the file back