editar /etc/netplan/00-installer-config.yaml

deste jeito para dhcp

network:
ethernets:
eth0:
dhcp4: true
version: 2

deste jeito para ip fixo
network:
ethernets:
eth0:
dhcp4: no
addresses: [192.168.0.7/24]
gateway4: 192.168.0.1
nameservers:
addresses: [192.168.0.1, 8.8.8.8]
version: 2