This tutorial shows how to add (or remove) the “proposed” repository to install (or downgrade) the pre-released packages/updates in Ubuntu.
As you may know, Ubuntu has an area called proposed
for updates that are being tested before publication. It’s recommended for developers and users who are interested in helping to test updates and provide feedback.
The proposed
repository is disabled by default. If you want, here’s how to enable it in all current Ubuntu releases.