- Write in a terminal:sudo apt-get update && sudo apt-get upgrade or edit Update Manager settings (try this first if you have already configured Update Manager).
- Launch Update Manager and try again.
Running update through a terminal gives option whether to allow or disallow untrusted packages.
WARNING: The following packages cannot be authenticated!
playonlinux
Install these packages without verification [y/N]?If you trust the package source, type y.
原文：http://askubuntu.com/a/117097
