I don't have the information if Ubuntu / Canonical 18.04 will be updated to 4.2.8p12 ntp. That would have to be filed with them.
However, you can install it by first downloading the newest version from http://support.ntp.org/bin/view/Main/SoftwareDownloads
Might need to install the build-essential as well for making the files.
sudo apt install build-essential
Then go to your Downloads folder and extract the file:
cd ~/Downloads
tar zxvf ntp-4.2.8p12.tar.gz
Go to the newly created folder:
cd ntp-4.2.8p12/
Now run the configure
, make
and sudo make install
./configure
make
sudo make install
The new version should now be installed. Reboot the system for the changes to take effect. After it is rebooted you can check for the newest version.
$ ntpd --version
ntpd 4.2.8p12@1.3728 Tue Mar 5 03:47:55 UTC 2019 (1)
Hope this helps!