What's the easiest way to install OpenSSH 7.4 on Ubuntu 16.04?

See instructions to upgrade SSH client to next possible Ubuntu version Yakkety 16.10 with provides SSH 7.3 (with Include config.d/* support):

echo "deb http://old-releases.ubuntu.com/ubuntu yakkety main" > /etc/apt/sources.list.d/yakkety.list
apt-get update
apt-get install -y ssh
rm /etc/apt/sources.list.d/yakkety.list
apt-get update

An alternative approach is installing the debian packages (although this approach is not ppa related)

https://gist.github.com/stefansundin/0fd6e9de172041817d0b8a75f1ede677

Just bear in mind that by doing this outside of apt then future installation management is messed up. To see what this would affect you can run

apt-cache rdepends openssh-client

and then going forwards when installing one those affected packages you will most likely see an error about the original version of openssh-client being missing . At least that is the behaviour I observed during the writing of some ansible scripts (Initially I upgraded the openssh-client last in the scripts to get around this)

In the error output in that scenario I noticed the output suggested to

apt-get -f install

without specifying a package. I did this, it installed a few bits (which apparently were not to do with openssh-client), and somehow future installs which depend on openssh-client now go through fine without complaining about the fixed version dependency.