I’d start from CentOS manuals, NethServer do not change CentOS.
https://access.redhat.com/documentation/en/red-hat-enterprise-linux/?version=6
NethServer network configuration is stored in a text-only database called networks, so to store/change a value the command is something like:
db networks setprop eth0 macaddr 11:22:33:44:55:66.
But I usually work/test the CentOS way and only when everything works I change the db if needed.