I cannot update or install softwares

[Yomal Amarathunge]

When i am going to install the updates using update Manager, a error messege comes and it says:
Requires installation of untrusted packages

The action would require the installation of packages from not authenticated sources.

I dont know how to correct this error.In previous weeks (before i updtae the update maneger programme) i was able to install any software and update.But now,the above error comes always
help me please
Description: Ubuntu 12.04.1 LTS
Release: 12.04

[Adolfo Jayme Barrientos (fitojb)]

Thank you for taking the time to report this issue and helping to make Ubuntu better. Examining the information you have given us, this does not appear to be a bug report so we are closing it and converting it to a question in the support tracker. We understand the difficulties you are facing, but it is better to raise problems you are having in the support tracker at https://answers.launchpad.net/ubuntu if you are uncertain if they are bugs. You can also find help with your problem in the support forum of your local Ubuntu community http://loco.ubuntu.com/ or asking at http://askubuntu.com or http://ubuntuforums.org. For help on reporting bugs, see https://help.ubuntu.com/community/ReportingBugs.