Proposed Archive provides pre-released updates for Ubuntu, by default it’s not enabled. There may be bug fixes or new features in pre-released update packages and you can selectively upgrade them by enabling proposed archive.

Enable Proposed Archive in Ubuntu 12.04 (11.04 and higher):

Launch Ubuntu Software Center, go to Edit -> Software Sources -> Updates tab, check “Pre-released updates (precise-proposed)”

Install Packages from Proposed Archive:

After enabling proposed archive, configure apt to allow selective installs of packages from it instead of upgrading all of your packages to the -proposed versions.

Open up a terminal(Ctrl+Alt+T, or search from dash), run command to create and edit this file:

sudo gedit /etc/apt/preferences

copy and paste following into the file and save it. With this preference file in place, Update Manager, Synaptic and Aptitude won’t ask for upgrades from the -proposed repositories.

Package: *
Pin: release a=precise-security
Pin-Priority: 990

Package: *
Pin: release a=precise-updates
Pin-Priority: 900

Package: *
Pin: release a=precise-proposed
Pin-Priority: 400

Now, install aptitude by:

sudo apt-get install aptitude

Use aptitude to list and install updates from proposed archive.

sudo aptitude -t precise-proposed

In aptitude, you should first update the package listings (‘u’ key), mark any of the packages you want to upgrade (‘+’ key), and finally install them (‘g’ key). After this, if you run Aptitude without options again, the rest of precise-proposed upgradable packages will remain hidden.

Alternatively, you can install a package from -proposed by using

sudo aptitude install packagename/precise-proposed

It is recommend that you re-enable Apport bug filing to capture and report crashes. In the event that you end up reporting a crash about a proposed package please tag the bug as proposed-pkg so we can distinguish it from other crash reports.

You can enable Apport on a stable system with:

gconftool -s /apps/update-notifier/show_apport_crashes --type bool true

… or if you use Kubuntu edit /usr/share/kubuntu-default-settings/kde-profile/default/share/config/adept_notifierrc and set Addept=true:

[General]
Adept=true

From Ubuntu Wiki