A Crouton Linux chroot update script
To update your chroot, first download the most recent crouton file to your Chrome OS Downloads directory. Then open the Chrome OS Shell and from your Downloads directory, run the command sh ucrouton chrootname,
where chrootname is the name of your chroot. Some examples are trusty, xenial and stretch. Check Crouton's documentation for details.
You may issue arguments to update multiple chroots, like this sh ucrouton trusty xenial.