Here's a more general way to revert from locally installed packages, proposed and backports packages to the stable+updates channel. While the other answers are correct and work in a similar way, I think this is a more elegant approach.

1. Make sure you removed the entries of -proposed or -backports in your /etc/apt/sources.list and /etc/apt/sources.list.d/* files.



2. Add an apt-preferences file, e.g. /etc/apt/preferences.d/99-back-to-stable-updates containing (for precise here - replace with your version):

   
   
   

   Package: *
   
   Pin: release a=precise
   
   Pin-Priority: 1001
   
   
   
   Package: *
   
   Pin: release a=precise-updates
   
   Pin-Priority: 1001
   
   
   
   Package: *
   
   Pin: release a=precise-security
   
   Pin-Priority: 1001
   
   
   
   Package: *
   
   Pin: release a=precise-proposed
   
   Pin-Priority: -10
   
   
   
   Package: *
   
   Pin: release a=precise-backports
   
   Pin-Priority: -10
   
   

   
   
   

   This pinning of > 1000 will make apt force a downgrade on packages from that channel and a priority of < 0 on the -proposed and -backports channels will make remove any additional packages too.




3. Now run

   
   
   

   sudo apt-get update
   
   sudo apt-get dist-upgrade
   
   

   
   
   

   Inspect the proposed solution apt will give you, and if you think it's okay, then accept it. If you need to check on why and what version it will be downgraded to, check this with apt-cache policy packagename to see what versions are available and what apt decides is the candidate for installation.




4. Remove the /etc/apt/preferences.d/99-back-to-stable-updates file again, as it's not needed anymore.

^{Thanks to mlind on the Ubuntu forums for this inspriation on pinning in his post from more than 5 years ago - yet it helped me out pretty well today.}