Ubuntu is the default distribution in crouton.

If no release is specified, precise is installed (Ubuntu 12.04 LTS). quantal (12.10), raring (13.04) and saucy (13.10) are also supported.

To upgrade to a more recent version of Ubuntu, you can use Ubuntu's update manager, or follow these instructions in a crosh shell:

• enter-chroot -n <chroot_name> (replace <chroot_name> by the name of your chroot, e.g. raring)
• sudo apt-get install update-manager-core python-apt
• do-release-upgrade

After the upgrade, it is important to make sure that crouton is updated:

• sudo sh -e ~/Downloads/crouton -n <chroot_name> -u

By default, the name of a chroot is its release name (e.g. a raring chroot's name is raring). After an upgrade, you end up with a saucy chroot, whose name is raring. Confusing? You can easily rename it with:

• edit-chroot -m saucy raring

LTS releases (e.g. precise, 12.04) are supported by Ubuntu for 5 years. By default, they will not upgrade to non-LTS release (e.g. quantal 12.10). If you really want to do that, edit /etc/update-manager/release-upgrades and replace Prompt=lts by Prompt=normal.