Change DNS servers in Ubuntu

Edit /etc/systemd/resolved.conf and change in:

[Resolve]
DNS=8.8.8.8
FallbackDNS=1.1.1.1

After that restart systemd-resolved daemon:

sudo systemctl restart systemd-resolved

As an quick alternative, you can change on-fly per interface:

sudo resolvectl dns wlp59s0 8.8.8.8

Note:
wlp59s0 is the interface name