How to change the time server for the _in built_ internet time in ubuntu 16.04 LTS?

Ubuntu 16.04 uses by default server ntp.ubuntu.com [reference].

To change the default server, edit the config file with an editor:

sudo vi /etc/systemd/timesyncd.conf

Uncomment the NPT= line and define the server you want to be used instead of default:

[Time]
NTP=some.ntp.server.com

To "audit" the time-synchronization events and verify the server that was contacted, use the following command:

cat /var/log/syslog | grep systemd-timesyncd

According to the official documentation at: https://help.ubuntu.com/lts/serverguide/NTP.html

The nameserver to fetch time for timedatectl and timesyncd from can be specified in /etc/systemd/timesyncd.conf and with flexible additional config files in /etc/systemd/timesyncd.conf.d/.