i'm beginner

More often than not, I've found that this error results from corruption in the security keys associated with the upgrade sources. The easy way to fix this is as follows (hit Enter after each line, and don't type the $ or # symbols):

$ sudo -i

# apt-get clean

# cd /var/lib/apt

# mv lists lists.old

# mkdir -p lists/partial

# apt-get clean

# apt-get update

Source: http://askubuntu.com/questions/1877/what-is-the-easiest-way-to-resolve-a...