How do I update sudo package version?

It is known as CVE-2019-14287 and already fixed as backport.

All you need - is to run:

sudo apt update
sudo apt upgrade

(but is more likely that unattended-upgrades was already installed all updates)

See USN https://usn.ubuntu.com/4154-1/ :

and was indicated in changelog of sudo 1.8.21p2-3ubuntu1.1 for 18.04 LTS.

For me it was enough to run:

1. sudo apt update
2. apt-cache policy sudo and check versions (Installed: 1.8.16-0ubuntu1.5, Candidate: 1.8.16-0ubuntu1.8 = this version fixed problem)
3. sudo apt-get install sudo

And the package was updated and check that bug: sudo -u#-1 whoami no more works