It's based on ubuntu 14.4 trusty, which is 2.5 years old at this point. Apparently beta versions use 16.4 xenial so that will be updated at some point. Howeve

It's based on ubuntu 14.4 trusty, which is 2.5 years old at this point. Apparently beta versions use 16.4 xenial so that will be updated at some point. However, it does not automatically update and the current recommendation is to save off anything you need, uninstall, and reinstall. Alternatively, you can sudo editing your /etc/apt/sources.list file to add an entry for xenial. I did that to replace hopeless obsolete versions of gcc (4 -> 5) and cmake (2.8 -> 3.2). So far, all of my problems with WSL have been ubuntu.